Review of City-Wide 30 km/h Speed Limit Benefits in Europe
Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsThe evidence of this paper is a quite valuable contribution to practitioners who want to implement safer speed limits, especially in developing countries around the world. I still see a few big limitations though that could be addressed.
1) The effects of speed limits in traffic speeds themselves are missing from this publication. More information could be provided by authors on the level to which speed reductions (due to the new city-wide 30 km/h speed limits) were effectively achieved after the speed limits imposition, and how the described benefits are attributable to speed reduction only. The pandemic effects should be more clearly differentiated from the speed limit reduction effects, for instance.
2) Were not there any negative effects documented from the speed limit reduction? I understand and agree that it is of interest to provide compelling evidence of the benefits to support this type of measures. However, a more comprehensive point of view of these type of measures demands an honest discussion on which negative implications we can observe - if any - from speed limit reductions. We all can agree that the positive effects far outweigh any possible negative effects, but this is an opportunity to expand this discussion.
3) From the point of view of public opinion, was it difficult to implement those 30 kph speed limit measures in certain locations? In some cases, groups of citizens could have opposed to the reduction of speed limits, and understanding the reasons they have will be useful to understand how to better expand this type of measures in the future.
4) How exactly has traffic congestion being measured? ¿Travel times, delays in traffic?
5) The average values of quantitative effects should be weighted by population to have a better idea of the overall effects among the cities with available data. For instance, in a large city -30% of X indicator was achieved and in a small town a -10% reduction was achieved, it would not be fair or accurate to express that there is simply a -20% average reduction.
Comments on the Quality of English LanguageOnly minor style revisions suggested.
Author Response
Please see the attachment.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
Comments and Suggestions for AuthorsThe paper is on an interesting topic.
The main problem with the paper is the lack of scientific rigor. There is almost no discussion of potential biases in the studies cited other than Covid. All studies cited seem to be accepted as equally valid. Many sources cited are popular press.
The paper is almost silent on actual speeds, enforcement, and speeds on surrounding roads. The paper makes it seem that speed limits are self-enforcing.
The authors really want lower speed limits to work. Their bias shows in many places throughout the paper. They also rely on adjectives to make points rather than citing credible data.
The strongest part of the paper is the section toward the end on research gaps. The authors should restructure the paper to make the point that we do not know much about the effects of lower speed limits yet, but lay out a research program to get to those answers.
Particular comments include:
Lines 47-55) Showing a bias in favor of the policy change. Also in Line 90 and elsewhere. Authors must be more balanced.
Line 61) No, lower speeds does those things, lower speed limits may or may not.
Line 98) Really big claim with no back-up or citation.
Line 145-150) The claims in this paragraph and others like it rely too much on adjectives like “slight”. Authors should quantify or should remove adjectives.
Line 152) The sentences about average speeds are misleading. So should cities encourage congestion to further reduce average speeds? Delete those sentences.
Line 167) How is anyone supposed to see a change in public health of any type in a short after period of some months or 1-2 years? Of a phased program for that matter?
Line 170) Everything hinges on the words “study quality”. What exactly were your parameters for that? The PRISMA effort only gets one to the studies that might be helpful, then the real work begins with assessing “study quality”. Condense the PRISMA description to a couple sentences and add at least a couple paragraphs on “study quality”.
Line 171-173) More adjectives!
Line 252) A five-month after period is subject to seasonal bias. Also, for this and other safety findings, was any effort made to account for other potential biases like demand changes or regression to the mean?
Line 257) The last quarter of 2020 was exactly in the middle of the Covid lockdown.
Line 273) Over what time periods, with what sample sizes, with adjustments for what biases? Same for the rest of that paragraph.
Line 285) Two weeks after? Including this finding casts doubt on any “study quality” represented in this paper.
Line 292) Covid effects?
Line 313) Good acknowledgement of potential Covid biases. Should have made that point more often, with each reported finding to which it could apply.
Line 347) Citing Vice? What happened to the emphasis on peer-reviewed journals? Too many citations from the popular press.
Line 367) Finally some data on speeds, but they do not seem credible. A 74% speed reduction does not seem possible.
Line 379) Other aspects can affect safety besides policy.
Line 381) Good point. That is called a halo effect, whereby drivers speed up outside of enforced areas to make up the time lost in enforced areas, and is well known in the safety literature. Is that the only effort you uncovered to study the halo effect?
Line 394) Poor writing. Editing needed in several places throughout the paper. Also, beginning at this line, why are paragraphs describing noise reductions under the heading “Emissions”?
Line 403) With all of the noise data, are those averages? Over how many samples? At what times of the day? The finding cited are not credible without more details on the methodologies.
Line 433) Do not capitalize “Opponents”. That makes it look like that is one organized group.
Line 443) Except the opponents would probably maintain that higher fuel efficiency occurs at somewhat higher speeds than 30 kmh.
Line 467) Is 30 sec per km really a “small” gap? Again, delete the adjectives.
Line 506) Is that the extent of your findings on health? If so, should drop all mentions of health effects from the paper except for a discussion of future research needs.
Line 581) Table 5 seems to be reporting average values from Table 3. That in turn implies that all findings in Table 3 are equally valid. Is that your claim? If so, need extensive justification.
Line 587) The discussion of research gaps is a good one. The authors should have sprinkled this discussion and some humility throughout the paper, not just at the end.
Line 599) How do you know that most of the effect was not due to Covid?
Line 640) I do not recall much discussion on enforcement or actual speeds. That would have helped in the body of the paper.
Comments on the Quality of English LanguageThe writing is decent, but errors occur throughout the paper and the paper needs a thorough edit.
Author Response
Please see the attachment.
Author Response File: Author Response.pdf
Reviewer 3 Report
Comments and Suggestions for Authors1. A well-written review paper. Solid methodology and nice summaries.
2. Some minor comments are:
3. Line 37: “In particular, particular emphasis is given safer vehicles …”. Change to “In particular, special emphasis is given to safer vehicles …”
4. Line 82: “The, the methodology of this study is then provided”
5. Line 260: “from 38 in the final three months of last year to 30 in the first quarter of 2021”. Change to “from 38 in the fourth quarter of 2020 to 30 in the first quarter of 2021”.
6. Line 262: “from 46 in the first quarter of 2020 to 30 at the same period in 2021”, change to “from 46 in the first quarter of 2020 to 30 in the first quarter of 2021”.
7. Line 298: “speed limit of 30 km/h on single-lane streets in each direction came into effect”. When did it come into effect?
8. Line 311: “not a single pedestrian or cyclist was killed in a road crash in 2020” change to “not a single pedestrian or cyclist was killed in road crashes in 2020”
9. Line 368: “including a negligible 1% reduction in fatal injuries”. Delete the word “negligible”.
10. Line 394: “Brussels in Belgium, established a citywide 30 km/h limit”. Add month and year of the implementation.
11. Line 395: “Five months after the implementation of 30 km/h speed limit,” delete “of 30 km/h speed limit”
12. Line 398: “Additionally, noise pollution decreased by 3 dB following the implementation of these limits”. Is this 3 dB the same as the 50% reduction? If so, delete this sentence and present as “50% (3 dB)”.
13. Line 415-416: rewrite this long sentence. Maybe “It was found that speed limits reduction led to a reduction in road traffic noise by ??? dB. Meanwhile, the speed limit reduction also had a positive effect on air pollution by nitrogen dioxide.”
14. Line 433-437: This has some logic problem. “study by Cerema (2020) which found that more CO2 emissions were generated at 30 km/h compared to 50 km/h.” But “according to the same study, vehicles produce significant polluting emissions when traveling at higher speeds”. Don’t they contradictory to each other? I guess the authors wanted to say that “the study said the 30 km/h generates more emission compared to 50 km/h ONLY when it is at constant speed with no acceleration and deceleration, but the reality is that the higher speed limit always means more acceleration and deceleration in urban areas. I suggest authors delete this whole paragraph because it only causes confusion rather than supports the topic.
15. “4.3. Energy” is much shorter than other sections, meaning that there are not too many quantitative studies on this subject. This is ok, but authors need to clearly state this and point out that more quantitative studies are needed. In addition, authors need to provide citations to each city’s conclusion (Brussels in Belgium, Munster Germany). There is zero citation right now. Again, “the general principles of traffic flow and vehicle efficiency at lower speeds”, you need a solid citation to support this important statement.
16. Line 448: “in July 2016, Edinburgh in UK has adopted a 20 mph speed limit”. Delete “has”.
17. Line 451: “city-wide” can be “citywide”.
18. Similarly, it seems more quantitative studies are needed for “4.6. Health”
19. Figure 2 is a great figure that summarizes this paper, so does Table 3.
20. Table 3: What does No (from 40 to 1) mean? What is the logic of the order in this table? Implementation time? City name alphabetically? Reference number? State this in Line 557.
21. Is Table 4 necessary? This is a large table. Yes, it contains great information that you have discussed previously, sources are in the reference, target goals are in Table 2. I would recommend adding “Implementation started” to Table 3, and delete Table 4 from this paper. Table 4 is redundant and too lengthy for a journal article.
22. Table 5 is a good summary. Maybe present the “Range” rather than “Max”.
23. Line 596: “safe-ty” should be “safety”.
24. “6. Conclusions” has a lot words about “Strategies for Implementation”. Maybe change to “6. Conclusions and Recommendations”.
25. Thanks for authors’ comprehensive review on this topic. Hope Greece can adopt this beneficial policy in the near future. Good luck!
Comments on the Quality of English LanguageSee above
Author Response
Please see the attachment.
Author Response File: Author Response.pdf
Round 2
Reviewer 2 Report
Comments and Suggestions for AuthorsThe paper is improved from the previous version. However, most of the evidence is still not from peer-reviewed sources, many potential biases still remain, and the authors' opinions are apparent throughout.
Comments on the Quality of English LanguageThe paper still has some errors and needs a thorough edit.
Author Response
Please see the attachment.
Author Response File: 
Author Response.docx
