The Impact of Speed Limit Change on Emissions: A Systematic Review of Literature

Abstract

In the pursuit of sustainable mobility and the decarbonization of transport systems, public authorities are increasingly scrutinizing the impact of travel speed on emissions within both low-speed and high-speed environments. This study critically examines the evidence concerning emission impacts associated with speed limit changes in different traffic environments by conducting a systematic review of the literature in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines. A total of 25 studies that met the eligibility criteria were assessed. The results reveal mixed evidence for reducing emissions through speed limit reductions in low-speed areas. However, emerging evidence suggests that reduced urban speeds may abate emissions through enhanced traffic flow and a shift in modal preferences away from personal vehicle use. Additionally, in urban areas, minor observed emission reduction per vehicle can add up to large overall reductions due to the high number of vehicles. In high-speed contexts, the evidence is much clearer, showing that reduced speed limits correlate with significant reductions in NOx, CO₂, and particulate matter emissions. The extent of these reductions is highly variable and contingent upon the specific speed limits or limit reductions, the local context, the vehicle type, and the baseline types and levels of pollutants. Notably, there is a lack of research on the effects of speed on emissions, especially in low- and middle-income countries (LMICs), highlighting a critical area for future investigation. The findings of this study underscore the potential environmental benefits of speed management policies and advocate for the promotion of smoother and less aggressive driving behavior to mitigate emissions and enhance sustainable mobility in both low-speed and high-speed settings.

1. Introduction

1.1. Background

There is a growing global concern regarding the effects of climate change on the environment and its impact on human health. Daily, numerous trips are made by households to meet their personal mobility needs, accessing facilities and services such as schools, offices, leisure, recreation, and public services [1]. However, many of these generated trips and mobility patterns are not sustainable in terms of emissions, nor in terms of safety and noise pollution. Consequently, countries are making efforts to promote sustainable mobility, particularly through means that reduce the emissions of greenhouse gases (GHGs) and other air pollutants generated during transportation. The goal is to transition to more environmentally friendly mobility.

Carbon dioxide (CO₂) constitutes a significant portion of GHGs, and recent statistics indicate that the transport sector experienced the most significant increase in CO₂ emissions, with a global surge of nearly 240 megatons in 2022 compared to 2021, and an annual average growth rate of 1.7% from 1990 to 2022, outpacing all other sectors [2]. This sector is also a major source of local air pollutants, emitting 20% of carbon monoxide (CO), 39% of nitrogen oxides (NOx), 26% of black carbon (BC), and 10% of particulate matter (PM) [3].

Given the significant contribution and rising trends in the transport sector’s emissions, the urgency to decarbonize the sector and mitigate the resulting ambient air pollution has never been more critical. Therefore, exploring innovative and effective strategies to reduce its environmental footprint is imperative to inform and guide policymakers.

Countries are adopting various policies and strategies to address this challenge. Some of these measures include limiting vehicle numbers and travel distances by promoting public and non-motorized transit; implementing traffic control measures like speed management, traffic light coordination, and signalization; enhancing the energy efficiency of vehicles through the adoption of electric or hybrid models; transitioning to alternative fuel sources; and motorization management [4,5].

Speed management policies, aimed at altering mobility patterns or driver behaviors, are acknowledged as effective for carbon abatement [6]. The relationship between speed, emissions, and fuel savings has been established for decades [7,8,9]. This relationship is often described to be U-shaped, signifying that there is an optimal speed range where deviations above or below this range lead to increased emissions and fuel consumption [10,11,12]. Historically, the practice of reducing speeds has been beneficial. For example, in the 1970s, the USA implemented a national maximum speed limit of 55 mph to conserve fuel, resulting in a 2% reduction in vehicle fuel consumption in 1974 and a 1.10% reduction in 1975 across all road types [8].

Speed limit reduction is just one of several ways to manage speed, along with other measures such as variable speed limits, Intelligent Speed Adaptation (ISA), speed cameras, and traffic calming measures (like speed bumps, chicanes, roundabouts, lane narrowing, and others). Drivers are also recognizing the need to adjust their attitudes towards speeding in order to reduce air pollution [13].

Nevertheless, there are often differential views on the impacts of speed levels (higher or lower speeds) on emissions. On one hand, it is argued that lower speeds could result in significant emissions due to factors such as idling, frequent gear changes, and more stop-and-go traffic conditions (i.e., frequent acceleration and deceleration) that generally consume a lot of energy (fuel) and hence increase emissions [14,15]. Additionally, at lower engine speeds, combustion heat loss is prolonged, allowing more time for pollutant emissions [16]; the reverse occurs for relatively higher speeds, where the shorter combustion time due to the mixture of fuel and air results in less available time for pollutant formation and emission [16]. On the other hand, higher speeds are considered important contributors to emissions because they rapidly increase friction, vehicle air resistance, and the frequency of high-combustion engine revolutions, leading to higher fuel consumption rates and emissions [16,17]. Nonetheless, the impact of speed on emissions, whether at low or high speeds, is said to depend on the type of pollutant, engine/vehicle type, and vehicle age/weight [5,10,18].

Compounding these discrepancies, studies on the effects of speed management on emissions also find heterogenous results, either in the magnitude of the effect (significant or insignificant) or in the direction of the effect. Additionally, the effects have been shown to differ when considering local/site or global/city level. For example, implementing isolated speed bumps to manage speeds has been attributed to increased local air pollution [19], while the effect of speed limit change has been observed to be widespread in both treated and untreated sites [20]. Important discrepancies have also been highlighted when using either macroscopic or microscopic traffic emission models to estimate changes in emissions due to speed changes [15]. Additionally, changes in emissions (such as CO₂) due to speed changes have been observed to not necessarily correlate with equivalent changes in ambient air pollutants (such as NOx, PM) [15].

Furthermore, there are likely to be differences between low- and middle-income countries (LMICs) and high-income countries (HICs) regarding the impact of speed change on emissions. The traffic in LMICs is often mixed, composed of different types of vehicles with varying dynamic characteristics, operating at significantly different speeds. This can lead to higher speed variations, which increases the frequency of acceleration and deceleration, with the consequences of increased emissions. In addition, LMICs often have older vehicle fleets than HICs, which generally have very high emission rates and are usually poorly maintained and not adequately regulated by public authorities. These factors could lead to variations in emissions between LMICs and HICs, but also differences in the anticipated benefits of emission reduction interventions such as speed management.

Considering these discrepancies, there is a need to understand the direction and magnitude of the effects indicated by robust studies of the effects of speed management on emissions. Consistent evidence from multiple robust studies could clarify the “true” impact of speed management/limitation on emissions.

While public authorities are keen to understand the effectiveness and co-benefits of travel speed changes on emissions and to establish speed management policies, they are often challenged by the perceived trade-offs of increases in travel time [21]. However, research on the impact of speeding or speed limit increases on travel time has shown only marginal travel time savings. In low-speed areas, the observed travel time savings are relatively small, such as 26 s per day [22], less than 1 min [23], and 1 to 5% time savings [24]. Similarly, in high-speed areas, the time savings range from 2 to 8% [25] and 1 s per kilometer [26]. Interestingly, some studies have even found that lowering speeds could potentially reduce travel time [27,28] and even more so for initiatives like variable speed limits (VSLs) [29,30].

Nevertheless, there are other compelling reasons for implementing speed management policies beyond environmental benefits. These are the expected benefits in terms of crash and trauma reductions and noise pollution reduction, given the links between speeds and these factors [21,31].

The effect of speed change on various transport externalities such as emissions, safety, travel time, congestion, or noise often depends on the initial speeds and differs when speed is reduced from a lower initial speed value compared to a higher one. Therefore, it is crucial to evaluate the impact of speed reduction on emissions, considering both low-speed environments (typically with speed limits < 80 km/h), such as urban arterials, collectors, feeders, or residential streets, and high-speed environments (typically with speed limits ≥ 80 km/h), such as rural highways, freeways, expressways, or intercity motorways).

1.2. Aims

This research addresses an important knowledge gap regarding the influence of speed management on vehicle emissions, which is vital for formulating sustainable mobility policies. There is currently no clear consensus in the existing literature on the effects of lower and higher speeds on emissions in terms of both direction and magnitude. This paper seeks to bridge this gap by evaluating the current body of scientific evidence regarding the impact of speed alterations (both reductions and increases) on emissions in both low-speed environments and high-speed environments. This is achieved through a systematic review of the existing literature on the impacts of different speed reduction mechanisms (such as speed limits) on emissions for different traffic environments. However, given that among all speed management measures, speed limit settings and revisions are almost ubiquitous and universally practiced, the study focused on looking at interventions involving speed limit changes, either in isolation or in combination with other measures. This measure is also cheap to implement and can allow policy makers to easily make informed decisions based on evidence.

This review contributes to both theory and practice. The systematic review will provide policymakers with evidence-based insights to inform the development of effective speed management policies. By focusing on the impact of speed limit changes, a common and cost-effective speed management measure, the study aims to clarify the “true” impact or effect of speed management on emissions across different traffic environments. This is particularly important for policymakers, who must balance the perceived trade-offs between travel time and environmental benefits. The findings and substantiated evidence from this study will serve as an instrumental resource, contributing to the development of policies that can enhance the sustainability of travel and overall mobility through emission reduction. Additionally, the study identifies research gaps in the literature and proposes recommendations for future policies and research in this area.

2. Materials and Methods

2.1. Protocol and Search Strategy

The literature review process adhered to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines [32]. The PRISMA framework provides an evidence-based, transparent, comprehensive, and replicable process for conducting a systematic literature search. We utilized the PRISMA 2020 (https://www.prisma-statement.org/prisma-2020-checklist (accessed on 6 August 2024)) check list, detailed in Supplementary Materials, as a structured tool to guide the overall process of searching the relevant materials and reporting the review across all sections including the abstract, introduction, methods, results, and discussions. This checklist was important in shaping our review protocol, defining the search strategy, establishing selection criteria, assessing the quality, and synthesizing the results.

The literature search was conducted between 1 October and 30 December 2023, and later updated in July 2024. The search strategy involved three main stages:

• Firstly, a comprehensive search was conducted using different bibliographic databases to retrieve published literature on the topic. These databases included Scopus, Google Scholar, ScienceDirect, and Research gates. A search query was created with the following limitations: published “year > 2000”; Document Type “Article or Review or Conference Paper”; Source Type “Journals”; and Language “English” or “French”. This broad search aimed to capture a wide range of potentially relevant literature.
• Secondly, a Google search was performed to identify websites with published reports or gray literature relevant to the study topic. This was supplemented by subject matter expert consultations to identify any additional publications that may have been missed in the initial search and are relevant for the review. This step helped to supplement the database search and ensure that the review included a more comprehensive set of relevant literature.
• In the final stage, the reference lists and bibliographies of the sources identified in the previous stages were thoroughly scrutinized to extract any additional relevant sources that may have been omitted in the previous stages. This supplementary reference list screening also helped to identify any relevant studies that were not indexed in the databases searched. The sources retrieved from all stages described above were considered relevant or irrelevant based on a pre-defined eligibility criterion (described in Section 2.1).

The literature search was initially conducted by an individual researcher and was later peer-reviewed by another researcher. This researcher supplemented the data, focusing primarily on literature retrieved from websites and through expert consultations. This approach ensured a more comprehensive and rigorous literature review.

The literature search through the databases was aided by a combination of several keywords related to the research topic, with the use of Boolean operators to combine and refine the search terms. The search terms used to retrieve the sources were as follows: (“Speed” OR “speed limit” OR “speed variations” OR “speed changes” OR “speed reduction” OR “speed increase”) AND (“vehicle emissions” OR “greenhouse gas” OR “traffic emissions” OR “pollution” OR “emissions” OR “Climate Change” OR “environment”) AND (“urban roads” OR “rural roads” OR “motorways” OR “expressways” OR “residential areas”). This comprehensive and systematic literature review process ensured that the study was based on a thorough and up-to-date understanding of the existing research on the relationship between speed and emissions across various road settings.

2.2. Eligibility Criteria

Before the literature search was conducted, an eligibility criterion (inclusion and exclusion) was defined to ensure the search was relevant to the research objectives, which aimed to identify the effects of speed limit changes on emissions. Numerous studies have evaluated the impact of various speed management measures on emissions, including posted speed limits, variable speed limits, and traffic calming (such as speed humps). However, the literature review specifically sought to synthesize evidence from studies focused on speed limit interventions across both low-speed and high-speed environments.

The inclusion and exclusion criteria for identifying and selecting relevant studies were as follows:

• Direct investigation of the impact of speed limit changes or mixed measures on vehicular emissions or pollutant emissions.
• Provision of quantitative or qualitative data on at least one of the following emissions or pollutants: nitrogen oxides (NO_x), carbon dioxide (CO₂), carbon monoxide (CO), and particulate matter (PM).
• Clear description of the study methodology or use of robust methodologies, including macro- and micro-modeling, simulation studies, on-road emission measurements, computational estimation, before-and-after evaluations, or a combination thereof.
• Publication in peer-reviewed journals, conference papers, or credible sources and websites to ensure the reliability of the findings.
• Studies in English or French, published after 2000.
• Excluded studies included studies with a lack of a clear comparative framework or absence of quantitative emission data/results; lack of full-text access or knowledge on methodologies employed for secondary sources; research not focusing on the direct relationship between real/simulated speed limits change and vehicular emissions; unclear methodologies for primary sources; and studies on emission rate and speed change models that are not focused on interventions (like speed limit change).

The quality of the included studies was ensured through a critical assessment of the methodologies employed and the overall reporting. This involved evaluating the data sources, study design and scope, controls, analysis methods, and the clear articulation of assumptions and limitations. Each study’s research design was assessed, examining whether it used controlled experiments, real-world observations, or simulations, and whether these methods were clearly articulated and justified.

Given the significant variations in study design and methodologies, strong emphasis was placed on assessing the overall transparency and reliability of the studies and the evidence reported. The clarity and completeness of the reporting in each study were examined, including the level of detail provided about data collection and analysis processes, as well as the disclosure of any potential conflicts of interest or limitations. Transparency is essential for enabling replication by other researchers and for readers to fully understand and trust the findings.

2.3. Screening, Documentation, and Analysis of Papers

The defined eligibility criteria guided the search, screening and documentation processes for the relevant studies and the removal of irrelevant ones. Each study was initially screened for relevance based on its title, abstract, and keywords. Subsequently, for the studies that passed this initial filter, the full-text documents were obtained and stored in a reference management application (Mendeley desktop).

The full-text documents were then thoroughly screened and examined to assess their compliance with the eligibility criteria and overall quality. Studies that did not meet the eligibility criteria were segregated into a separate folder, while the remaining relevant papers were used for further analysis.

After identifying and reviewing all the included studies in their entirety, information on the general characteristics of each paper and the findings regarding the impacts of speed limits on emissions were extracted. The data extracted on the study characteristics included the authors, year of publication, geographical location where the study was conducted, and the methodology employed.

The findings on the impact of speed limits on emissions were classified according to two themes: low-speed environments (defined as locations with speed limits < 80 km/h, such as urban arterials, collectors, feeders, or residential streets) and high-speed environments (defined as locations with speed limits ≥ 80 km/h, such as rural highways, freeways, expressways, or intercity motorways). Within each theme, data were extracted on the speed limit change values and the impact (magnitude and direction) on emissions/pollutants of nitrogen oxides (NOx), carbon dioxide (CO₂), carbon monoxide (CO), and particulate matter (PM).

The search and documentation process are summarized and presented according to the PRISMA flow chart in Figure 1. This flow chart provides a clear and standardized visual representation of the study selection process for the research. It illustrates the various stages of the review, including the initial identification of potentially relevant studies and removal of duplicates, the subsequent screening for eligibility based on predefined criteria, the assessment of full-text articles for inclusion, and the final number of studies that were ultimately included in the synthesis.

3. Results

3.1. General Results of Literature Search

The literature search resulted in 1831 papers after the removal of duplicates. Following the screening of abstracts, 78 papers were identified (including those sourced from bibliographies) for full-text review. The eligibility assessment, guided by predefined inclusion and exclusion criteria, yielded 25 records for synthesis. Of these, 15 studies focused on high-speed environments, while 12 addressed low-speed environments; two studies encompassed both.

The articles retrieved spanned from 2008 to 2023, with a fluctuating number of publications per year. The years 2008, 2011, 2012, and 2020 witnessed a modest peak of three or four publications, while the remaining years exhibited one or two publications. The distribution of publications did not exhibit a clear trend over the years, indicating a sustained interest in the subject matter.

The geographic distribution of the literature was varied, with Spain being the most prolific, contributing six publications. The United Kingdom followed with four, and Germany, Belgium, Ireland, the Netherlands, and Norway each contributed two publications, while China, France, Hungary, Sweden, and the United States each had one. These data suggest a predominantly European-centric research landscape on the topic and limited research in LMICs.

3.2. Review Findings on the Impact of Speed Limits Interventions on Emissions

3.2.1. Synthesis of Studies in Low-Speed Environments

Table 1. Synthesis of studies on the relationship between speed and emissions in low-speed environments.

Authors	Measures	Methods	Effects	NOx	CO2	CO	PM	Location/Year
Introduction of 30 km/h speed limits (20 mph)
[15]	Reducing speed limits for passenger vehicles from 50 to 30 km/h	Macro- and micro-modelling	Only minor reductions in NOx and CO2 (inconsistent among methods) PM may increase or decrease (results dependent on vehicle type)	↓	↓		↓↑	Local roads. Belgium and Spain 2011
[33]	Speed limit reduction from 50 to 30 km/h	A simulation study	Reductions in CO2 by 26.8% Reduction in NOX by 26.7%	↓	↓			Urban residential areas in Antwerp, Belgium 2011
[34]	Introduction 30 km/h speed limits	A simulation study accounting for changes in traffic demand	CO2 reductions in 3/3 cities NOx reductions in 2/3 cities (with one negligible increase) PM reductions in 2/3 cities (one modest increase) Emission reductions were clearly dependent on the reduction in kilometers of travel. Emissions increased when only speed reduction was accounted for.	↓	↓		↓↑	Dresden, Magdeburg, Stuttgart, Germany 2023
[35]	Influence of 30 km/h speed limit on emissions compared with 50 km/h.	Based on on-road emission measurements of a light vehicle with a turbo-diesel engine and oxidizer catalyst	Findings showed that 30 km/h speed limit driving resulted in: 27% NOx reduction 21% CO reduction 22% PM reduction compared with 50 km/h speed limit driving. Also showed a 15% reduction in fuel consumption.	↓		↓	↓	Madrid, Spain 2012
[36]	Hypothetical decrease in speed limits 50 to 30 km/h	A simulation study	NO2 decreased by 40% PM10 decreased by 10%	↓			↓	Urban network in Berlin, Germany 2022
[37]	Comparison of sites with 30 mph and 20 mph speed limits	Evaluation based on vehicle trials	Higher emissions of NOx (+7.9%) and CO2 (+2.1%) pollutants in petrol vehicles in 20 mph zones Lower emissions of NOx (−8.2%) and CO2 (−0.9%) pollutants for diesel vehicles in 20 mph zones Lower particulate matter emissions (−8.3%) for all vehicle types in 20 mph zones.	↓↑	↓↑		↓	Urban roads in London, UK 2013
[38]	Hypothetical decrease in speed limits 50 to 30 km/h	A simulation study	CO increased by 21% HC increased by 22% CO2 increased by 8% NOx increased by 12%	↑	↑	↑		Urban network in Budapest, Hungary 2021
[39], [40]	Hypothetical decrease in speed limits 50 to 30 km/h	A simulation study	NO2 increased by 1 to 13% (2020) NOx increased by 3% (2019) PMx increased by 2% (2019, 2020).	↑			↑	Urban network in Dublin, Ireland 2019, 2020
[41]	Hypothetical 20 mph speed zone comparison with 30 mph zones	Computational estimation study (no data collected)	NOx emission increased by 7.6% (deaths due to NOx increased by 63) PM10 decreases by 24.9% (deaths decreased by 117)	↑			↓	Urban areas in Wales, UK 2017
Mixed measures
[35]	Influence of driving styles on emissions in Madrid	Based on on-road emission measurements of a light vehicle with a turbo-diesel engine and oxidizer catalyst	Findings showed that eco-driving style resulted in: 22% NOx reduction 11% CO reduction 56% PM reduction Compared with Aggressive driving style. Also showed a 24% reduction in fuel consumption.	↓		↓	↓	Madrid, Spain 2012
[42]	0.5 × 0.5 km 20 mph zones using signage and speed humps	Controlled before-and-after study based on onsite emission measurements	In one zone, concentrations of NO2 decreased at all sites, including the control, by between 4% and 13%; Concentrations of benzene decreased by 10−35% at all sites including the control. In a second zone, NO2 concentrations increased by 1–10% at all sites including the control. Concentrations of benzene increased at all sites, including the control (19–36%). NB: Changes were not significant (p > 0.05)	↓↑				UK, NW England 2005
[43]	Traffic calming measures on key streets in Leeds (road narrowing, 20 mph signs, new shared road surface)	Study from meta-analysis (unknown)	Little change for benzene and NO2 before and after; the control site and one intervention site showed slight decrease (−5% and −10%, respectively), the 3 other intervention sites showed increased (2–43%) Relative to the control site, benzene concentration decreased slightly at intervention sites. NB: All findings were non- sig.	↓↑				Urban areas in Leeds, UK 2020

NB: ↑ Increase, ↓ decrease, ↓↑ increase or decrease.

summarizes the effects of speed reduction on emissions from 12 studies relating to 20 mph/30 km/h speed limit reduction analysis and mixed speed limit reduction/traffic calming measures. The synthesis of research on the impact of speed limit interventions on emissions in low-speed environments presents a mixed picture, with some consistent trends but also notable variations depending on the specific methods, locations, and vehicle types considered.

Ref. [15] observed only minor reductions in nitrogen oxides (NOx) and carbon dioxide (CO₂) emissions when speed limits for passenger cars were reduced from 50 to 30 km/h, with particulate matter (PM) emissions showing inconsistent changes. In contrast, another study reported substantial reductions in CO₂ (26.8%) and NOx (26.7%) emissions following a speed limit reduction from 50 to 30 km/h in urban residential areas of Antwerp, Belgium [33]. A more recent simulation study by Schmaus et al. [34] found more consistent reductions in CO₂, NOx, and PM emissions in three German cities (Dresden, Magdeburg, and Stuttgart) when speed limits were reduced to 30 km/h. However, these reductions were contingent upon a decrease in total kilometers driven.

Ref. [35] reported significant reductions in NOx (27%), carbon monoxide (CO, 21%), and PM (22%) when comparing driving at 30 km/h versus 50 km/h. Ref. [36] simulated a speed limit reduction from 50 to 30 km/h and found a 40% reduction in NO₂ and a 10% reduction in PM10 emissions in Berlin, Germany, suggesting that lower speed limits could enhance air quality.

In contrast, other studies indicate that lowering speed limits to 30 km/h or 20 mph can lead to increased emissions. For example, a simulation study in the urban network of Budapest, Hungary [38], and a study in Dublin, Ireland [41], both reported increases in CO, hydrocarbon (HC), CO₂, and NOx emissions when speed limits were decreased. These findings highlight that in some contexts, lower speed limits may inadvertently lead to increased emissions.

The effects of speed limit reductions on emissions appear to be highly dependent on the specific context, including factors such as the urban road network, vehicle fleet composition, and driver behavior. For instance, a study in London, UK [37], found that while diesel vehicles showed reductions in NOx and CO₂ emissions in 20 mph zones, petrol vehicles exhibited higher emissions of these pollutants. Additionally, the impacts of driving style on emissions have been highlighted by other authors [35], who found that an “eco-driving” style resulted in significant reductions in NOx, CO, and PM compared to an “aggressive” driving style, irrespective of the speed limit.

Studies in the UK [42,43] showed mixed results for the impact of 20 mph zones, with some areas experiencing decreases in NO₂ and benzene, while others saw increases. These findings suggest that the benefits of low-speed zones may not be consistent across different locations and may be influenced by other factors such as traffic flow and congestion.

3.2.2. Synthesis of Studies in High-Speed Environment

The synthesis of research on the impact of speed limit interventions in high-speed areas (

Table 2. Synthesis of studies on the relationship between speed and emissions in high-speed environment.

Authors	Measures	Methods	Effects	NOx	CO2	CO	PM	Location/Year
Speed limit reductions
[44]	Introduction of 80 km/h speed limits or variable speed system where speeds were at 100 km/h and 120 km/h	Simulation study for variable speed limit	Reduced NOx by 5.7% Reduced CO by 5.1% Reduced SO2 by 4.8% Reduced PM10 by 5.1%	↓		↓	↓	Metropolitan motorways in Barcelona, Spain 2008
		Simulation study for 80 km/h speed limit	Reduced NOx by 1% Reduced CO by 1% Reduced SO2 by 0.9% Reduced PM10 by 0.9%	↓		↓	↓	Metropolitan motorways in Barcelona, Spain 2008
[46]	Speed limit reduction from 120 to 80 km/h	Simulation and modelling	Reduced NOx by 4%	↓				Swiss motorways 2008
[5]	Introduction of 80 km/h speed limits where speeds were at 100 km/h and 120 km/h	Before/after evaluation	Reducing emissions by 4–11% Improving air quality by 10–15% Reduction by 14.81% for CO, 10.98% for nitrogen oxides, 12.47% for PM2.5 and 10.99% for PM10	↓		↓	↓	Metropolitan motorways in Barcelona, Spain 2010
[45]	Introduction of 80 km/h speed limits or Variable speed system where speeds were at 100 km/h and 120 km/h	Further evaluation regressions for variable speed limit	Reduced NOx by 7.7–17.1% Reduced PM10 by 14.5–17.3%	↓			↓	Metropolitan motorways in Barcelona, Spain 2013
[47]	Reducing speed limits from 110 km/h to 90 km/h	Macroscopic traffic simulation	Road sections with high decrease in mean speed experienced daily savings in emissions by 2 to 10% One of the sections with a lower decrease in mean speed experienced an increase in emission.	↓	↓		↓	Lille motorway in France 2014
[12]	Increasing * speed limit by 10 mph (16 km/h) from 55 to 65 mph	Before/after evaluation	Increase in CO by 23% Increase in NO2 by 15% Increase in O3 by 11% No sig. change in PM10 (* direction of change reversed for comparison with speed reduction studies)	↓*	↓*	↓*	↓↑	US freeways 2015
[20]	Speed limit reduction from 90 to 70 km/h	Simulation based study	For treated roads: 16.4% reduction in NOx 14.4% reduction in CO2.	↓	↓			Inter-urban roads, Madrid, Spain 2017
		Simulation based study	For overall road section (treated & untreated) 4.6% reduction in NOx 4.1% reduction in CO2	↓	↓
[34]	Introducing nationwide maximum speed limit of 120 km/h	Microscopic traffic flow simulation model combined with travel demand modelling and emission modelling	Reduction of 9.6% in NOx Reduction of 4.2% in CO2 Reduction of 6.6% in PM With kilometers of travel having a lower impact that for lower speed ranges evaluated.	↓	↓		↓	Intercity motorways in Germany 2023
	As above combined with 80 km/h speed limit outside of urban areas	Microscopic traffic flow simulation model combined with travel demand modelling and emission modelling	Reduction of 11.1% in NOx Reduction of 5.1% in CO2 Reduction of 7.3% in PM	↓	↓		↓	Intercity motorways and rural roads in Germany 2023
[48]	Speed limit reduction from 100 km/h to 80 km/h	Before after evaluation	Reduced PM10 by 7.4% Reduced PM1 by 2.8% Reduced black smoke by 15% No. stats. sig. effect on NOx reduction Reduction in non-intervention areas also observed for PM10 and black smoke.	↓↑			↓	Amsterdam: urban ring highway 2023
[49]	Speed limit reduction from 80 to 60 km/h (effects near treated roads)	Evaluation	No improvement in air quality Weak evidence for increase in NOx	↓↑			↓↑	National road, Oslo, Norway 2023
[50]	Environmental speed limits (80 km/h reduced to 60 in winter, then returned to 70 km/h or 80 km/h in summer)	Modelling using speed data	Low to negligible effects on PM2.5, NOx and CO2 Reduction in PM10 emissions by 6–12%	↓↑	↓↑		↓	Metropolitan area of Oslo, Norway 2020
[15]	Reducing speed limits for trucks from 90 to 80 km/h	Macro- and micro-modelling	CO2 emissions decrease (9%) NOx & PM increase slightly (2 & 4% respectively)	↑	↓		↑	Motorways in Belgium and Spain 2011
Mixed speeding interventions
[53]	Intelligent Speed Adaptation (ISA) project on 70 mph roads (125 km/h)	Measurements/observed changes:	Reduced CO2 emissions by 5.8% on 70 mph roads Insignificant effects on low-speed roads		↓			UK roads 2012
[51]	80 km/h speed limit with strict enforcement	Scenario modelling and estimations	Reduced PM10 emissions by non-significant to 8% Reduced NOx emission by 30 to 32% Note: Stronger results were observed for uncongested traffic.	↓			↓	Motorways in Rotterdam and Amsterdam, The Netherlands 2010
[52]	Analysis of speed limit enforcement regimes Toronto vs. Beijing (stronger regime with lower speeds, low speeding, smoother traffic flow).	Modelling of standardised GPS speed data, with Beijing showing comparative	14% reduction in CO2 57% reduction in CO 14% reduction in NOx, and 21% reduction in Particle Numbers	↓	↓	↓	↓	Urban roads in Toronto and Beijing. 2012

NB: ↓* justifies that if an increase in speed leads to increasing emissions, then the lower speeds would have reduced emissions; ↑ Increase, ↓ decrease, ↓↑ increase or decrease.

) indicates that reducing speed limits generally leads to a decrease in vehicle emissions. Studies conducted in metropolitan motorways in Barcelona, Spain, consistently found reductions in NOx, CO, SO₂, and PM10 emissions with the introduction of 80 km/h speed limits [5,44,45]. Similarly, Ref. [20] observed significant emission reductions on inter-urban roads in Madrid when speed limits were lowered from 90 to 70 km/h. Other authors also reported a decrease in CO₂ emissions, albeit with slight increases in NOx and PM, when speed limits for trucks were reduced from 90 to 80 km/h in Spain [15]. Ref. [46] found comparable results on Swiss motorways, with reductions in NOx after speed limits were decreased from 120 to 80 km/h. A French case study also noted emission savings on motorways when speed limits were reduced from 110 km/h to 90 km/h [47]. Studies have also shown that increasing speed limits leads to emission increases. For example, a study in the US showed that increasing speed limits increased the emissions of CO, NO₂, and O₃, providing a counterexample to the benefits of speed reductions [12]. A German study showed that the introduction of a nationwide maximum speed limit of 120 km/h on intercity motorways (highways) led to reduced emissions of NOx, CO₂, and PM, with further reductions achieved when an 80 km/h limit was enforced outside urban areas [34]. Ref. [48] reported a decrease in PM10 and black smoke on an urban ring highway in Amsterdam following a speed limit reduction from 100 km/h to 80 km/h, although no significant impact on NOx was detected. Mixed outcomes were reported in Norway for a speed limit reduction from 80 to 60 km/h, where no improvement or a potential increase in NOx emissions was observed, and effects on PM2.5 and CO₂ were low to negligible [49,50]. Studies have also reported significant reductions in PM10, NOx, and other emissions when speed limit settings were combined with strict enforcement [51,52]. Interventions such as Intelligent Speed Adaptation (ISA) on 70 mph roads in the UK were also observed to reduce CO₂ emissions [53].

4. Discussion

The findings from various studies underscore the impact of speed limit interventions on enhancing sustainable mobility by reducing emissions. As expected, the effects differ based on the speed environment, i.e., whether it is a low-speed or high-speed environment. This is because the impact of increasing or decreasing speed on factors such as emissions, safety, or travel time would depend on the initial speed that was changed.

In low-speed environments, studies have shown conflicting results regarding the potential impacts of speed limit reductions on emissions. While the majority of studies indicate a probable decrease in pollutant emissions, a few reports show an increase, and some show uncertain or negligible changes. The magnitude of these effects varies across studies, depending on the range of speed limit reductions, the type of pollutant, accompanying interventions, and the methodologies employed.

It is crucial to note that most studies were based on models or simulations of speed limit changes, which relied on vehicle emission rate models that typically predict higher fuel consumption at lower speeds, which were associated with frequent braking and acceleration. Changing these rate models to recognize more even speed profiles in traffic, calmed by lower speed limits, would result in more realistic results. This remains to be investigated.

Most studies showing a likely decrease in emissions often reflect only a minor to moderate impact, and cases where an appreciable impact is observed are limited to the emission benefits of specific vehicle types. For example, evidence from some studies suggested that emission reductions at low speeds could be obtained for some individual vehicle types (depending on their fuel type and dynamic characteristics in traffic)—such as diesel vehicles—and not for others [15,35]. This brings an overall uncertainty regarding the mean reduction in emissions in low-speed areas when all vehicle types in traffic are considered. Conducting studies that consider all vehicle types and their driving characteristics in traffic might be challenging, but such comprehensive analyses will be essential to clarify the expected overall benefits in emission reduction.

It is important to note that although studies in low-speed areas may indicate only a slight decrease in emissions, or a reduction for specific vehicle types, even small per-vehicle changes can result in significant overall emission reductions. This is because small per-vehicle reductions, when multiplied across the large volume of urban vehicles and the cumulative effect over a year, can lead to appreciable emissions decreases. Consequently, managing speeds in low-speed zones can lead to a meaningful reduction in emissions, which contributes to the positive impact of speed management.

Some studies suggest that aggressive acceleration and deceleration are primary contributors to increased emissions in low-speed areas. Therefore, enhancing the smoothness of acceleration and deceleration maneuvers at low speeds could be more effective in reducing emissions than simply lowering the average speed [37]. Although this review did not primarily focus on these factors, evidence from various studies indicates that traffic flow approximating eco-driving (smooth and steady), the implementation of eco-variable speed limits, and consistently low-speed traffic calming schemes that eliminate abrupt and frequent stop-and-go movements are crucial for reducing emissions in urban areas, as opposed to the current speed-limitation-only strategies [24,54,55].

Nevertheless, while the direct benefits of speed reduction in low-speed environments remain ambiguous and require more understanding, accounting for indirect benefits such as a decrease in motorized travel reveals significant emission reductions. For instance, one of the studies reviewed attributed the appreciable reductions in emissions to diminished vehicle travel demand due to the speed limit reduction, rather than the reduction in mean speed itself [34].

Moreover, lower speeds tend to encourage road users to adopt more sustainable modes of transportation, such as walking and cycling. This modal shift leads to a decrease in motorized travel, which is a primary source of emissions. One study demonstrated that walking and cycling increased by up to 12% following the introduction of 20 mph zones [56]. Nonetheless, studies that consider these indirect effects of speed limit reductions are often scarce.

In high-speed environments, the evidence supporting emission reductions through speed or speed limit reductions is considerably more consistent. The majority of studies show significant environmental benefits in terms of reductions in emissions and air pollutants when speeds are reduced from higher levels (120, 100, 90 km/h) to moderate speeds, like 60–80 km/h, which emerge as the most optimal speed limit range in high-speed environments. Only a few studies have reported negligible or context-dependent results.

The evidence suggests that the size of the effects of speed limit reduction on emissions depends on the initial amounts of pollutants [45]. However, the variation in study findings may also be attributed to the varying methodologies used, such as those based on models or simulations of real or proposed changes, which could be prone to underlying assumptions about emission rate models and other modeling assumptions. In addition, given that the majority of the research is European-centric, especially in Spain, the findings for this speed environment may be influenced by local or context-specific conditions of European countries.

Across all the studies reviewed, one potential explanation for the divergent findings on the effect (direction and magnitude) of speed limits on emissions could be the influence of other measures or contextual/local factors beyond just speed limit changes. The results reported for mixed measures in Table 1 and Table 2 generally indicate that the effectiveness and sensitivity of emission reduction (or increase) caused by speed limit changes can be affected by the presence and configuration of other measures, such as enforcement and physical traffic-calming interventions. For example, evidence from various studies suggests that when lower speed limits are combined with other interventions in either high-speed or low-speed environments, the overall emission reduction is more significant compared to isolated speed limit changes, although there are some discrepancies for certain pollutants in low-speed areas. Greater reductions in emission magnitudes have been reported when speed limit measures were complemented by strict enforcement in high-speed areas [51,52]. Furthermore, studies that compared variable speed limits (VSLs) with fixed speed limits showed that VSL introduced in high-speed environments were more effective in reducing emissions than fixed speed limits [44]. This is perhaps attributable to their effects on traffic dynamics, especially in congestion management, including reduced acceleration and deceleration.

Conversely, in low-speed environments, the effects of speed limit changes, whether implemented alone or in combination with other measures, were less definitive for some pollutants. This observation could be partly attributed to variations in driver behavior and compliance with speed limits in different study contexts. If these behavioral changes are not accounted for in studies, the results could vary significantly. This highlights the importance of analyzing the effects of speed limit changes by separating them from the influence of other concurrent measures or by controlling for their effects. Understanding the independent impact of speed limit changes, as well as their interaction with other contextual factors or speed management strategies, such as infrastructure modifications, enforcement, intelligent transportation systems, and vehicle technology improvements, is vital for future research. However, the evidence available suggests that speed limit changes alone may not consistently be an effective measure for emission reduction and should be part of a broader set of complementary interventions that influence travel demand, driver behavior, and the overall traffic system. Policymakers should view speed limit modifications as one element of a holistic approach to sustainable transportation, rather than as a standalone solution.

Notably, in both low-speed and high-speed environments, there is limited research conducted in LMICs. The reviewed literature revealed that none of the included studies addressing low-speed areas were conducted in LMICs, and only a single study was identified for high-speed areas within an LMIC context. This absence of LMIC-specific research represents a significant knowledge gap, as traffic composition and behavior can vary markedly between HICs and LMICs.

In addition to the identified gaps, it is noteworthy that none of the studies have evaluated the impact of a shift from internal combustion engines (ICEs) to electric vehicles (EV), a movement that will undoubtedly have an impact on vehicle emissions. In addition, there is no content on speed changes on the energy-related emissions of EVs. Higher speed limits or driving at higher speeds typically necessitates greater energy consumption for EVs, leading to an elevated demand for electricity and more frequent recharging. Depending on the type of power generation and the availability and location of the power grid or charging facilities, this could result in either an increase or decrease in emissions tied to electricity generation. This highlights a potential area for future research to explore how speed limits affect the energy-related emissions of electric vehicles.

5. Conclusions and Future Implications

This review was conducted to evaluate the existing literature and evidence regarding the impact of speed limit interventions on emissions in both low-speed and high-speed settings. The findings indicate different levels of certainty in evidence between these two environments.

In low-speed environments, the evidence is mixed, with most studies showing a decrease in pollutant emissions in response to speed limit reductions, while some report mixed or increased emissions. However, when considered, the indirect benefits of speed limit reduction such as reduced travel demand due to modal shift may lead to overall emission reductions in low-speed areas. Most studies estimated emission effects using models that do not reflect the calmed traffic characteristics found with low speed limits.

In high-speed environments, the evidence supporting emission reductions through speed or speed limit reductions is considerably more clear. The majority of studies show significant environmental benefits in terms of reductions in emissions and air pollutants when speeds are reduced from higher levels to moderate speeds.

However, for each road environment, the effects are context-dependent and vary based on factors such as the magnitude of speed limit reduction, the type of pollutant, the vehicle characteristics, accompanying interventions, and the research methodologies employed across studies.

As next steps, it is clear that reducing speed limits in both high-speed and low-speed environments has the potential to reduce emissions, hence supporting the implementation of speed-limit-reduction policies to enhance sustainable mobility. In addition to reducing speed limits, cities could consider adopting measures that promote smoother driving (eco-driving), to reduce acceleration and deceleration, and technologies (in-vehicle technologies and feedback signs) that inform drivers of their speed behavior or force them to comply to speed limits. The implementation of fixed speed limits with rigorous enforcement and other supporting measures such as traffic calming, ISA, VSLs, and eco-driving may be more effective at reducing speeds and emissions than speed limit reductions alone. Therefore, policymakers are encouraged to not only pursue speed limit reductions but also to enhance strategies to improve compliance.

However, it is important to note that most of the research to date has been conducted in HICs, leaving a significant knowledge gap that needs to be filled regarding the impact of speed limit reductions in LMICs, where traffic scenarios are often more heterogeneous.

There is also a pressing need for the development of standardized speed–emission-rate models that accurately represent diverse traffic scenarios including calmed low-speed traffic, stop–start arterials, and free-flow traffic. Existing models also need to be critically assessed and reviewed. Models applicable to the diverse vehicle types in LMICs, including those with different fuel types, are also critically needed. Such models would be instrumental in assessing better estimates of the effects of speed management on emissions and the associated health impacts on the population.

Further research is necessary to fully understand the benefits of lower speeds, such as the potential for modal shifts and the impact of smoother driving patterns on emissions, particularly in the varied traffic conditions of LMICs. In addition, research on the impacts of speed limit on energy related emissions for electric vehicles should also be explored. Finally, future reviews should also aim to fully synthesize the impact on emissions of physical speed calming measures such as speed humps, speed bumps, speed tables, chicanes, etc. Such studies should aim to synthesize the effective of collective schemes and not individual treatments.