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HFG SPEED PERCEPTION, CHOICE, AND CONTROL Version 1.0 EFFECTS OF POSTED SPEED LIMITS ON SPEED DECISIONS Introduction The effects of posted speed limits on speed decisions refers to the impact that posted speed has on actual speeds selected by drivers. This guideline covers light-vehicle driver compliance with posted speed limits on non-limited- access rural and urban highways. Drivers are legally in compliance when they are traveling at or below the posted speed limit. At a practical level, however, drivers are typically given--and they expect to be given--some small margin above the posted speed limit before being subject to law enforcement (1). Driver compliance is best assessed under free-flow conditions for a roadway segment because driver speed behavior is then largely unconstrained by external influences (e.g., traffic congestion, road work, or extreme weather) and they are free to choose their "natural" speed based on the roadway. Design Guidelines Posted speed limits should not be used as the only method to limit free-flow speed in light vehicles. For most urban and rural highways, increasing or decreasing the posted speed limits changes 85th percentile speed by approximately 1 to 2 mi/h in the same direction as the change. For interstate freeways, increasing the posted speed limits increases 85th percentile speed by approximately 1 to 3 mi/h. Speed dispersion also increases. The figure below shows daytime traffic speed distributions and illustrates driver-selected speed relative to posted speed, as well as overall speed dispersion. The data are from interstate highways in Montana, both before (1995 data, 55 mi/h posted speed) and after (1996 data, at least 70 mi/h posted speed) the repeal of the National Maximum Speed Limit (NMSL) law (effective December 8, 1995). 40 JAN, FEB, MAR PERCENT OF DRIVERS 30 1995 20 1996 10 0 23 28 33 43 48 53 58 63 68 73 78 83 88 MPH 40 APR, MAY, JUN PERCENT OF DRIVERS 30 1995 20 1996 10 0 23 28 33 43 48 53 58 63 68 73 78 83 88 MPH 40 JUL, AUG, SEP PERCENT OF DRIVERS 30 1995 20 1996 10 0 23 28 33 43 48 53 58 63 68 73 78 83 88 MPH Source: recreated from Milliken et al. (2) Based Primarily on Based Equally on Expert Judgment Based Primarily on Expert Judgment and Empirical Data Empirical Data 17-8
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HFG SPEED PERCEPTION, CHOICE, AND CONTROL Version 1.0 Discussion It is quite clear from both everyday observation and existing research data that most drivers do not comply with posted speed limits. In Harkey, Robertson, and Davis (3), data were collected and analyzed from 50 locations in four states to determine travel speed characteristics. The authors reported that 70.2% of drivers did not comply with posted speed limits, specifically (1) 40.8% exceeded posted speed limits by more than 5 mi/h; (2) 16.8% exceeded posted speed limits by more than 10 mi/h; and (3) 5.4% exceeded posted speed limits by more than 15 mi/h. Milliken et al. (2) conducted a broad review of current practices in setting speed limits and provided guidelines to state and local governments on appropriate methods of setting speeds limits and related enforcement strategies. With respect to driver perceptions of speeding and speed limits, the review found that (1) most drivers do not perceive speeding as a particularly risky activity; (2) most drivers will drive at what they consider an appropriate speed regardless of the speed limit; and (3) advisory speeds have modest to little effect on driver speed, particularly for drivers who are familiar with the road. Taken together, these attitudes result in generally low compliance with posted speed. Also from Milliken et al. (2), changing speed limits does not always result in the intended changes in behavior. Lowering the speed limits on major highways reduced both travel and speed fatalities, although driver speed compliance gradually eroded. Drivers violate new, higher speed limits because they expect the same enforcement tolerance of 5 to 10 mi/h at the higher limits. Specifically, average and 85 th percentile speed typically increased 1 to 3 mi/h despite larger increases in the speed limit--a minimum of 5 mi/h. Parker (4) also found that increasing or reducing the posted speed on urban and rural non-limited access roadways did not significantly change the number of injury or fatal crashes. Overall, changes in speed limits seem to simply legalize existing driver behavior; that is, they change compliance levels rather than speeding behavior. The findings suggest the difficulty of altering behavior merely by changing a speed sign. As noted elsewhere, speed choices are clearly mediated by a number of factors. Milliken et al. (2) found evidence that speed enforcement is the most common mediator between speed limit and speed choice. Where speed choice is not constrained by speed limits and their enforcement, the driver does trade off travel time and safety. In an analysis of FHWA data, Uri (5) found that adherence to the 55 mi/h limit does depend on the time cost of travel, cost in terms of discomfort and irritability, enforcement and, for a subset of states, the price of gasoline. Design Issues One design issue to consider when changes to the posted speed limit are contemplated is the possibility of speed changes carrying over to connecting roadways. The basic idea is that drivers adapt to higher speeds on the primary road and will be biased toward driving at those higher speeds once they switch to a connecting roadway. The evidence for carryover effects is limited, especially because many studies find such a small relationship between posted speed limit change and free-flow speed on the principal roads (4, 2). Another issue that may be worth examining in detail is the effects of speed limit changes on speed dispersion. Speed limit changes increase speed dispersion on interstate freeways, and variation in drivers' speed appears related to crash risk (2). Cross References Speeding Countermeasures: Setting Appropriate Speed Limits, 17-10 Speeding Countermeasures: Communicating Appropriate Speed Limits, 17-12 Speeding Countermeasures: Using Roadway Design and Traffic Control Elements to Address Speeding Problems, 17-14 Key References 1. Giles, M.J. (2004). Driver speed compliance in Western Australia: A multivariate analysis. Transport Policy, 11(3), 227-235. 2. Milliken, J.G., Council, F.M., Gainer, T.W., Garber, N.J., Gebbie, K.M., Hall, J.W., et al. (1998). Special Report 254: Managing Speed: Review of Current Practice for Setting and Enforcing Speed Limits. Washington, DC: Transportation Research Board. 3. Harkey, D.L., Robertson, H.D., and Davis, S.E. (1990). Assessment of current speed zoning criteria. Transportation Research Record, 1281, 40-51. 4. Parker, M.R., Jr. (1997). Effects of Raising and Lowering Speed Limits on Selected Roadway Sections. (FHWA-RD-92-084). McLean, VA: FHWA. 5. Uri, N.D. (1990). Factors affecting adherence to the 55 mph speed limit. Transportation Quarterly, 44(4), 533-547. 17-9