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5  Speed Limit Relationships and Practices Speed and Crashes Approximately one-quarter of all traffic fatalities are related to speeding (Figure 2), either traveling in excess of the posted speed limit or driving too fast for the conditions. Although the downward trend is encouraging, speeding continues to be a primary contributor in traffic fatalities. Ongoing Debate on How to Set a Posted Speed Limit Several sources are available to aid in evaluating and identifying the appropriate posted speed limits. Many states and cities have their own laws and criteria for setting of speed limits, with some being more detailed than others. The Manual on Uniform Traffic Control Devices for Streets and Highways (MUTCD) (4) provides details on the use of speed limit signs as a traffic control device (TCD), providing details on color, size, retroreflectivity, etc. The MUTCD also provides general advice on variables to consider when selecting the speed limit on a roadway segment; however, specific methods and decision steps are not included. The MUTCD broadly serves as a reference regarding the setting of speed limits; other references and guidelines to provide more detailed criteria for selecting the posted speed limit. This user guide provides such a procedure to calculate suggested speed limits. Many different approaches are available and used to set a posted speed limit. Within the United States, the operating speed approach based on the 85th percentile speed is typically used. In the operating speed approach, the selection of the speed limit value uses the measured 85th percentile speed for the roadway segment, and in some cases, adjustment factors that con- sider a number of conditions are also applied. The driver often plays a key role in the speed limit setting process since the speeds con- sidered when establishing speed limits are typically measured when traffic is flowing freely. During free-flow conditions, drivers select speeds that they believe optimize the tradeoffs between travel time and risk. Basing the speed limit on the 85th percentile indicates a belief that drivers are pretty good at assessing these tradeoffs, and that their judgment is trustworthy in establishing a level where exceeding that speed may be cited by law enforcement. While that may be true, additional conditions could exist that do not influence the 85th percentile speed but do contribute to crashes. A posted speed limit that is lower than the 85th percentile speed could help to minimize the consequences of those conditions. In addition, the desire to pro- vide roadway corridors that encourage active (non-motorized) transportation should con- sider the safety and mobility needs of pedestrians and bicyclists when setting posted speed limits. Given these competing preferences, the debate about the best approach to setting speed limits is ongoing. S E C T I O N 2
6 Posted Speed Limit Setting Procedure and Tool: User Guide This user guide discusses a procedure that can be used to identify a suggested posted speed limit for a street or highway segment. The procedure is based on the speed distribution for a segment of current drivers with adjustments for the consideration of safety. The Consequences of Speed The release of the recent National Transportation Safety Board (NTSB) report Reducing Speeding-Related Crashes Involving Passenger Vehicles (5) provides insight into the ongoing challenges related to speeding and examines the causes and trends in speeding-related passenger vehicle crashes along with countermeasures that can prevent these crashes. Such issues include driver speed behavior and the setting of speed limits, data-driven approaches for speeding countermeasures and enforcement, and the use of automated speed enforcement as a deterrent. The report reflects the understanding that addressing speeding involves a continuum of design approaches, countermeasures, and policies all aimed at supporting a community safety plan. It is well known that speed has an influence on crash severity, particularly in pedestrian crashes, and evidence shows that speed may also influence the number of crashes. The severity increases are not linear with respect to speed and tend to increase more substantially at higher speeds. It is unclear whether knowledge of that on the part of drivers would influence their speed choice. Some transportation professionals and safety experts believe that the 85th percentile should not be the sole factor in determining the speed limit, particularly in urbanized areas. For example, it may be prudent to post speed limits that are lower than the 85th percentile on road- ways with pedestrians and/or bicyclist activity. However, if the decision is not based on objective data or accompanied by needed enforcement, education, or infrastructure changes, then slower travel speeds may not be achieved. Drivers often make their personal speed assessment based on their own needs and perceptions and do not necessarily consider other road users. Challenges with the Relationship Between Posted Speed and Operating Speed Establishing speed limits is often a complicated task. If speed limits are set with safety as the only consideration, the result will be low speed limits, which is not practical for mobility. The speed limit is generally a policy decision made by elected or appointed officials, typically 0 5 10 15 20 25 30 35 0 2000 4000 6000 8000 10000 12000 14000 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 S pe ed F at al iti es a s Pe rc en t o f T ot al To ta l S pe ed in g- R el at ed F at al iti es Year Fatalities Percent Source: Data from Insurance Institute for Highway Safety, âFatality Facts 2017: Yearly Snapshotâ (3). Figure 2. Motor vehicle crash deaths involving speeding as a contributing factor, 2008â2017.
Speed Limit Relationships and Practices 7  after considering the recommendations of their agencyâs traffic engineers but not always, and sometimes without limiting their considerations to 85th percentile speeds. Like most efforts in traffic engineering, setting speed limits involves balancing competing desires and perceptions. One key issue facing the profession is what measurable factors should be considered in making these recommendations and their respective weights pertaining to speed limit. In addition, the process should incorporate the consideration of safety. Consideration of which roadway and roadside characteristics to include in the decision- making process is central to the discussion related to speed. As illustrated with data for urban streets in Figure 3, the existing average operating speed is closer to the posted speed limit than the 85th percentile speed. This supports the observation that the setting of posted speed limits is influenced by more than the 85th percentile speed. Possible factors affecting speed (and safety) include, but are not limited to: ⢠Crash history including severity consequences. ⢠Available roadside elements. ⢠Horizontal curvature characteristics including radius, superelevation, and friction. ⢠Roadway lighting. ⢠Adjacent pedestrian and bicycle activity. ⢠Roadway facility type and context. ⢠Number of signals. ⢠Number of access points. ⢠Type of median. ⢠Presence of sidewalk. ⢠Presence of bicyclist facilities. The linear trendlines in Figure 3 demonstrate a relationship between the posted speed limit and the operating speed. The average and 85th percentile operating speeds are higher when the posted speed limits are higher, or are lower when the posted speed limits are lower. While several roadway characteristics also influence operating speed, the research conducted in this project found that the posted speed limit influences operating speed (2), indicating that the number on the sign does matter. Several other studies have also found the posted speed limit y = 1.0153x + 3.752 R² = 0.7765 y = 0.9527x + 0.4807 R² = 0.7416 10 20 30 40 50 60 70 20 25 30 35 40 45 50 55 O pe ra tin g Sp ee d (m ph ) Posted Speed Limit (mph) Urban Streets 85th Percentile Speed Average Speed Posted Speed Limit = Operating Speed Linear (85th Percentile Speed) Linear (Average Speed) Figure 3. Comparison of operating speeds versus posted speed limits on urban streets.
8 Posted Speed Limit Setting Procedure and Tool: User Guide has a significant effect on free-flow speed on urban streets (6, 7, 8, 9, 10, 11, 12), rural two-lane highways (13, 14, 15, 16), and rural multilane highways (17, 18). In addition to the safety impacts of speed limits, another area of substantive debate is how much speed limits influence the actual speed selection behavior of drivers. Research has gen- erally shown that speed limit changes result in changes in the observed mean and 85th per- centile speeds but are less pronounced than the actual speed limit changes. This has been true for cases where speed limits were decreased (19, 20) or increased (21, 22, 23, 24, 25). In one of the most extensive studies in this area, Parker (26) conducted a large-scale study from 1985 to 1992 to determine the impact that raising or lowering posted speed limits on non-Limited-Access highways had on driver behavior. At the time of the study, the maximum speed limit on such roadways was 55 mph. Over the duration of the study, states and local authorities raised and lowered posted speed limits on short segments of roadways, typically less than 2 miles in length. Data on driver behavior and crashes were collected from 22 states. These included 100 sites along non-Limited-Access highways where the speed limits were either raised or lowered and 83 control sites where speed limits were not changed. The range of speed limit changes consisted of lowering the speed limit by 5, 10, 15, or 20 mph, or increasing the speed limit by 5, 10, or 15 mph, with only one change made at each site. Interestingly, the difference in operating speed after these changes was less than 1.5 mph on average (26). Kockelman (13) found that speed limit increases tend to increase average vehicle speeds. On average, speed increases were generally less than half the amount of the actual speed limit increase. Dixon et al. (27) reviewed speed data for 12 rural multilane sites in Georgia to evaluate the effects of repealing the 55-mph national speed limit. The authors found that operating speeds were higher after the increase in the posted speed limit. The evidence cited in the NTSB report (5) also indicates that speed limits do have some effect on operating speed, primarily in increasing them and perhaps in reducing them to a lesser extent. The magnitude of the change in operating speed when there is an increase (or decrease) in posted speed is typically only a fraction of the amount of the actual speed limit change (13, 28, 29, 30). For undivided high-speed rural roadways, mean speeds are generally 3 to 5 mph higher for every 10-mph increase in speed limit above 55 mph, with smaller increases at higher speed limits (13, 28, 29). In summary, while the research findings indicate a change in the posted speed limit sign can affect operating speeds, it is not as influential as the magnitude of the speed limit value change. If traffic engineers could actually achieve desired operating speeds merely by setting and post- ing speed limits, their work would be done. Simply setting speed limits without other corrective measures is rarely likely to achieve target speeds, which is the operating speed intended for drivers to go on a given roadway facility. Granted, setting appropriate speed limits is an essen- tial step in achieving target speeds, so it is critical to improve the methods for recommending them. The overwhelming reality is that there will never be enough law enforcement resources to enforce speed limits, no matter how they are determined. Furthermore, it will require a fundamental change in public opinion before automated enforcement (spot or segment) is adopted on a broad basis. Achieving Target Speeds Through Roadway Configuration and Traffic Control The central issue to achieving target speeds involves the configuration and operation of road- ways so that target speeds, compatible with context and all roadway users, are chosen byâ and not forced uponâvehicle operators. However, much of the roadway context, especially the
Speed Limit Relationships and Practices 9  urban one, has already been established, so a large part of the effort of achieving target speeds involves retrofitting the existing environment. Since only lane width, reallocating the cross section, elements on the roadside such as bus stops or trees, and vertical and horizontal deflec- tions to alter the physical alignment are available, a clear understanding of what combination of those, and in what configurations, achieves target speeds (or at least what greatly influences operating speeds) is needed. Several previous research efforts (31) and anticipated research efforts (32) offer insights, but a formula for achieving a target speed is currently not available. Transportation professionals can install the simplest and most straightforward, proven method to achieve target speeds on major streets in urban areas by implementing traffic signal progression. If drivers realize they will have a stop-free, steady, but appropriate speed to travel, then they may be more likely to actually drive the posted speed. For low-speed urban roads and streets that are unsignalized, transportation professionals will have to achieve target speeds through appropriate combinations of physical design features, many of which are now being included in context-sensitive, complete streets.
