Skip to main content
You can also sort pages by filters.
Table of Contents
Download the Full Book

Effectiveness: 3 Star Cost: $
Use: High
Time: Varies

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). The Rosen and Sander (2009) study estimated fatality risk curves based on driver impact speeds, ranging from 8% at 50 km/h (31 mph) and reaching 50% at 75 km/h (about 47 mph).

Reducing speeds through lowering speed limits is a time-honored countermeasure. Evidence shows, though, that actual speeds are reduced by only a fraction of the reduction in speed limits – typically 1-2 mph speed reduction for every 5-mph speed limit reduction. However, even 1-2 mph reductions in average speed are estimated to yield substantial fatal and injury crash reductions over all, with higher percentage reductions on streets with lower initial speeds (AASHTO, 2010, Table 3E-2; and see the Speeding and Speed Management chapter). Speed affects pedestrian injury severity to an even greater degree, as mentioned above and earlier in the chapter, and consequently also affects pedestrians’ perceptions of whether it is safe to walk. 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). 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. Speed limit reductions need to be made compelling through communications strategies (framing the problem), appropriate engineering changes such as road diets (typically modifying a four-lane roadway to three lanes, with the middle lane becoming a turn lane), traffic calming, and roundabouts, and by speed enforcement (including by automated means). On roads intended for higher speeds, measures that separate pedestrians from traffic as they travel along the road (on sidewalks), or cross the road (such as median refuges and signals like pedestrian walk signals or leading pedestrian intervals, that provide pedestrians opportunities to cross) should be provided and are also keys to safer environments (Howard et al., 2008). For more on speeding-related issues and countermeasures, see the Speeding and Speed Management chapter. Combinations of these measures can improve road safety and are increasingly used as part of the Vision Zero initiative in many cities in the United States (Kim et al., 2017; Vision Zero Network, 2018).

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. If done cleverly, this can be accomplished with relatively modest engineering changes and expense. As mentioned above, “road diets,” a proven safety measure, may be a low-cost way to reduce a “big, wide” street that suggests high speeds to drivers and also provide more space for pedestrians, bicyclists, or on-street parking. (For more information, see­WhitePaper_RoadDiets_PBIC.pdf for a review of road diets and safety effects and FHWA’s Road Diet Informational Guide, As of January 2018 many cities including Boston, New York, Seattle, 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 (Vision Zero Network, 2018).

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, six of which had pavement treatments simulating traffic calming devices. Although no pedestrian crash reductions were observed in the police districts with the program compared to those without, pedestrian crashes were too small in number to achieve measurable effects. For more information, see the Speeding and Speed Management chapter, Sections 1.1, 2.2, and 4.1.

Use: High, in the sense that all public roads have a speed limit and speed limit enforcement is widely employed.

Effectiveness: Reduced speed limits with enforcement can reduce vehicle speeds and all types of crashes and crash severity. The association of pedestrian injury with speed trends strongly suggests that pedestrian injuries and crashes will be reduced if travel speeds are reduced. This association is currently being examined by communities that lowered citywide speed limits. In January 2017 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 September to November 2017 found statistically significant reductions of 2.9%, 8.5%, and 29.3% in the odds of vehicles exceeding 25 mph, 30 mph, and 35 mph on the city streets. 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).

Costs: 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 can be 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 a year to plan and implement a complex plan.

Other issues:

  • 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 citizen buy-in, which involves a clear understanding of the reasons for the change.