Lower Speed Limits

The goal of reducing motorist travel speeds is to increase reaction time for both drivers and pedestrians to avoid crashes, as well as reduce the severity of pedestrian injuries when these crashes occur. Higher vehicle speeds produce more frequent and more serious pedestrian crashes and casualties, as evidenced by several studies (Leaf & Preusser, 1999; Rosen & Sander, 2009; Tefft, 2011; Martin & Wu, 2018; Thomas et al., 2018). Rosen and Sander (2009) estimated fatality risk curves based on driver impact speeds, ranging from 8% at 31 mph and reaching 50% at about 47 mph. Motorist speed affects pedestrian injury severity and consequently also affects pedestrians’ perceptions of whether it is safe to walk. For a more thorough discussion of speed-related issues please see the chapter on Speeding and Speed Management. Reducing and enforcing speed limits is just one tool among many for decreasing travel speeds with the goal of improving pedestrian safety. For more information, see NCHRP Synthesis 595 Pedestrian Safety Relative to Traffic-Speed Management (Sanders et al., 2019).

Speed limits are in effect on all road segments in all States. It is important to keep in mind that actual travel speeds often exceed posted or statutory speed limits and that posted speed limits often exceed safe travel speeds. Evidence shows that actual speeds are reduced by only a fraction of the reduction in speed limits—typically 1- to 2 mph speed reduction for every 5-mph speed limit reduction (Elvik et al., 2004). However, even 1- to 2 mph reductions in average speed are estimated to yield substantial fatal and injury crash reductions. Some reasons that travel speeds do not decrease by the same proportion as speed limit reductions, include drivers not noticing the new speed limit, drivers not understanding the safety reasons to reduce speed, drivers speeding out of habit, or continuing to keep up with the speeds maintained by other drivers. For maximum effectiveness, speed limit reductions need to be accompanied by communications and outreach that inform the public and make the case for the speed reduction, and by heightened, visible enforcement (Leaf & Preusser, 1999). Speed limit reductions need to be made compelling through communications strategies (framing the problem); appropriate engineering changes such as road diets, lane narrowing, traffic calming, and roundabouts; and by speed enforcement (including automated). For information on automated enforcement, see the PBIC document An Overview of Automated Enforcement Systems and Their Potential for Improving Pedestrian and Bicyclist Safety (Poole et al., 2017). Combinations of these measures can improve road safety and are increasingly used as part of Vision Zero initiatives in many cities in the United States (Kim et al., 2017; Vision Zero Network, 2018). Cities including Boston, New York, Seattle, Minneapolis, and Portland, have implemented the Vision Zero framework to improve pedestrian safety and survival chances by lowering speed limits on roads with a high frequency of multimodal traffic. Case studies and webinars featuring these communities are available on the Vision Zero Network website.

Speed limit reductions can be most effective when introduced to a limited area as part of a visible area-wide change, for example, identifying a downtown area as a special pedestrian- friendly zone through signs, new landscaping or “streetscaping,” lighting, etc. As mentioned above, road diets, an FHWA proven safety countermeasure, may be a low-cost way to reduce an overbuilt street that suggests high speeds to drivers and provide more space for walking, bicycling, and for drivers who need to park their vehicles.

If speed limits are routinely ignored, then enforcing speed limits may be a more effective strategy than attempting to change them. Blomberg and Cleven (2006) reported on demonstration programs in two cities in which speed limit enforcement, combined with engineering changes and extensive publicity, reduced both average speeds and the number of excessive speeders in residential neighborhoods. One attempt to scale up a similar program to a large city (Philadelphia) met with challenges in garnering community involvement and increasing enforcement due to a State restriction on using radar to enforce speeds, and seemed to have limited success in reducing injuries (Blomberg et al., 2012). However, speed reductions were observed on 17 of 24 corridors, 6 of which had pavement treatments simulating traffic calming devices. As with many evaluations of countermeasures for pedestrian safety, pedestrian crashes were too few to achieve measurable effects.

Use:

High, in the sense that all public roads have a speed limit and speed limit enforcement is widely employed, even if it is not with the express purpose of improving pedestrian safety.

Effectiveness:

Reduced speed limits with enforcement can reduce vehicle speeds and all types of crashes and crash severity. However, the measure of effectiveness for most studies of speed limit reductions or enforcement operations is speeds, not crashes or severity of injuries. An example of a crash-based study is an evaluation of “slow zone” treatments in London, England, which found a reduced risk of crash severity for pedestrians when urban speed limits are low (e.g., 20 mph) (Li & Graham, 2016). Since the association of pedestrian injury with speed trends strongly suggests that pedestrian injuries and crashes will be reduced if travel speeds are reduced, and pedestrian crashes can be a relatively rare event compared to other motor vehicle crashes. Most studies examine changes to average speeds, 50th percentile speeds, 85th percentile speeds, or excessive speeding. In January 2017 the City of Boston lowered speed limits on city streets to 25 mph from 30 mph. The speed reductions were heavily publicized on traditional and social media (Hu & Cicchino, 2020). An analysis of vehicle speeds during the months of September-November 2017 found statistically significant reductions in the odds of motorists exceeding 25 mph (2.9% lower), 30 mph (8.5% lower), and 35 mph (29.3% lower) on Boston streets compared to streets in a nearby control city. The effect of enforcement was not studied in this analysis; however, the city had installed speed limit and speed feedback signs at certain locations (Hu & Cicchino, 2020). Research conducted in Canada suggests that speed limit reductions may not affect excessive speeding behaviors (Heydari et al., 2014). Both studies reinforce the importance of looking at measures of speed, not just over/under the speed limit.

Cost:

Simply changing speed limits is low-cost, only requiring updating speed limit signs or, where few signs exist, adding some new ones. Combining speed limit changes with communications and outreach, enforcement, and engineering changes is significantly more expensive.

Time to implement:

Depending on the scope of the program, the time can be very short, or it can take several months to more than a year, especially if legislative changes are needed.

Other considerations:

Speed limit changes exist in the context of other, unchanged speed limits. The normal expectation is that there is an overall consistent approach to speed-limit setting. Where, for safety, some speed limits need to be reduced in a manner inconsistent with other speed limits, there must be clear and visible reminders that distinct conditions exist that justify the lower limits. Also, speed limit changes can be more effective if there is public buy-in, which involves a clear understanding of the reasons for the change.