Recently, the rapid increase in traffic volume has led to traffic jams on expressways, affecting driving safety and efficiency [
1]. China is a mountainous country, and the profile designs of its roads need to overcome the country’s severe terrain, thus complicating vertical alignment [
2]. Road geometry has great influence on road safety and traffic efficiency [
3]. On uphill sections of mountainous expressways, heavy vehicles need to decrease their speed to overcome greater slope resistance. The mixed traffic of passenger and freight vehicles with large speed differences can reduce the driving freedom of overtaking, further resulting in lower traffic efficiency, especially on four-lane freeways [
4]. Evacuation and rescue are more difficult once congestion or an accident happens on mountainous freeways [
5]. Therefore, it is of great importance to study the vehicle driving performance on uphill sections of mountainous freeways for relieving traffic pressure and safe driving. In this paper, speed reduction refers to the reduction in speed of trucks below the average running speed of traffic, revealing the mutual influence between large and small vehicles. In vertical alignment design, speed reduction is used to determine the critical length of grade. Climbing lanes are advantageous when excessive speed reductions are anticipated [
6]. Hence, it makes sense to study the effect of speed reduction on traffic efficiency in order to improve the traffic conditions. Overall, the study of speed reduction in relation to traffic efficiency has important practical significance on uphill sections of mountainous freeways.
The American Association of State Highway and Transportation Officials (AASHTO) Policy on Geometric Design of Highways and Streets currently employs a 15-km/h speed reduction criterion [
7]. The 15-km/h regulation was chosen from the Texas Highway Department’s 1968 speed survey [
8]. The crash involvement rate increased rapidly for increases in speed reduction beyond 15 km/h, which was applied to establish an objective basis for the speed-reduction criterion [
8]. In this survey, only safety factor was considered. Traffic efficiency is affected greatly by speed reduction, especially on the uphill sections of mountainous freeways [
2]. Traffic efficiency refers to the measurement of kilometers of vehicle transportation completed by the expressway facilities per unit of time [
9]. However, the existing knowledge about speed reduction and traffic efficiency is very limited [
7,
8], as the researchers currently pay more attention to speed dispersion [
10,
11,
12,
13,
14,
15,
16,
17,
18,
19,
20]. Numerous studies have reported positive correlations between estimated traffic crash rate and speed dispersion [
10,
11,
12,
13,
14,
15,
16,
17,
18]. Speed dispersion has two definitions: the average range of speed difference between two neighboring vehicles and the standard deviation of individual speed [
14,
15]. It can reveal interactions between adjacent vehicles. Research related to speed dispersion has been conducted regarding road design, traffic flow characteristics, and crash rate. Speed dispersion with the first definition was found to be exponential with density, exponentially negative with mean speed, and two-phase linear to flow [
15]. In a specified congested state, flow rate will decrease with an increase in speed dispersion [
14]. The first definition of speed dispersion is better in describing traffic flow status. Speed dispersion can influence traffic efficiency with larger average speed [
16]. The research scope of speed dispersion has been determined, but the specific value hasn’t been given. The positive association between the standard deviation of speed and crash rate has been verified [
17] and the findings are basically consistent with the Green Book [
18]. However, the positive correlations between traffic crash rate and speed dispersion have been questioned by some scholars [
19,
20]. They have claimed that the observation of these correlations provides no support for the hypothesis that increases in speed variance increase individual risk. Overall, the relationship between traffic crash rate and speed dispersion has been extensively studied. By contrast, it is difficult to obtain satisfactory reports about speed reduction and traffic efficiency from the relevant research at present. Speed dispersion focuses on speed distribution but it cannot directly reflect the interaction between a truck and passenger car; examining speed reduction can achieve this goal. This paper aims to find reasonable speed reduction concerning traffic efficiency to determine the critical length of grade, thus improving the profile design of roads. After comprehensive consideration, speed reduction was chosen as the research variable in this paper.
To the best of our knowledge, there is still no specific research at present which investigates the effect of speed reduction on traffic efficiency, especially in mountainous areas. The existing studies have mainly concentrated on the relationship between speed reduction and crash involvement ratio [
7,
8]. To understand the impact of speed reduction on traffic efficiency with different vertical slopes, a field experiment was firstly carried out. Platoons were then selected based on the car-following theory. By applying the data analysis software Origin, the relationship between speed reduction and traffic efficiency was fitted. By comparative analysis, the variation law of traffic efficiency with speed difference was obtained. The advantages of this research are two-fold. Firstly, the optimum speed reductions for different vertical slopes were proposed, which can help determine the critical length of grade. Secondly, this study fills the research gap in speed reduction from the traffic efficiency perspective.