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Human Factors Guidelines for Road Systems: Second Edition (2012)

Chapter: Chapter 17 - Speed Perception, Speed Choice, and Speed Control

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Suggested Citation:"Chapter 17 - Speed Perception, Speed Choice, and Speed Control." National Academies of Sciences, Engineering, and Medicine. 2012. Human Factors Guidelines for Road Systems: Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22706.
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Suggested Citation:"Chapter 17 - Speed Perception, Speed Choice, and Speed Control." National Academies of Sciences, Engineering, and Medicine. 2012. Human Factors Guidelines for Road Systems: Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22706.
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Suggested Citation:"Chapter 17 - Speed Perception, Speed Choice, and Speed Control." National Academies of Sciences, Engineering, and Medicine. 2012. Human Factors Guidelines for Road Systems: Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22706.
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Suggested Citation:"Chapter 17 - Speed Perception, Speed Choice, and Speed Control." National Academies of Sciences, Engineering, and Medicine. 2012. Human Factors Guidelines for Road Systems: Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22706.
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Suggested Citation:"Chapter 17 - Speed Perception, Speed Choice, and Speed Control." National Academies of Sciences, Engineering, and Medicine. 2012. Human Factors Guidelines for Road Systems: Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22706.
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Suggested Citation:"Chapter 17 - Speed Perception, Speed Choice, and Speed Control." National Academies of Sciences, Engineering, and Medicine. 2012. Human Factors Guidelines for Road Systems: Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22706.
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Suggested Citation:"Chapter 17 - Speed Perception, Speed Choice, and Speed Control." National Academies of Sciences, Engineering, and Medicine. 2012. Human Factors Guidelines for Road Systems: Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22706.
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Suggested Citation:"Chapter 17 - Speed Perception, Speed Choice, and Speed Control." National Academies of Sciences, Engineering, and Medicine. 2012. Human Factors Guidelines for Road Systems: Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22706.
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Suggested Citation:"Chapter 17 - Speed Perception, Speed Choice, and Speed Control." National Academies of Sciences, Engineering, and Medicine. 2012. Human Factors Guidelines for Road Systems: Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22706.
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Suggested Citation:"Chapter 17 - Speed Perception, Speed Choice, and Speed Control." National Academies of Sciences, Engineering, and Medicine. 2012. Human Factors Guidelines for Road Systems: Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22706.
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Suggested Citation:"Chapter 17 - Speed Perception, Speed Choice, and Speed Control." National Academies of Sciences, Engineering, and Medicine. 2012. Human Factors Guidelines for Road Systems: Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22706.
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Suggested Citation:"Chapter 17 - Speed Perception, Speed Choice, and Speed Control." National Academies of Sciences, Engineering, and Medicine. 2012. Human Factors Guidelines for Road Systems: Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22706.
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Suggested Citation:"Chapter 17 - Speed Perception, Speed Choice, and Speed Control." National Academies of Sciences, Engineering, and Medicine. 2012. Human Factors Guidelines for Road Systems: Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22706.
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Suggested Citation:"Chapter 17 - Speed Perception, Speed Choice, and Speed Control." National Academies of Sciences, Engineering, and Medicine. 2012. Human Factors Guidelines for Road Systems: Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22706.
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Suggested Citation:"Chapter 17 - Speed Perception, Speed Choice, and Speed Control." National Academies of Sciences, Engineering, and Medicine. 2012. Human Factors Guidelines for Road Systems: Second Edition. Washington, DC: The National Academies Press. doi: 10.17226/22706.
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Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Behavioral Framework for Speeding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .17-2 Speed Perception and Driving Speed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .17-4 Effects of Roadway Factors on Speed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .17-6 Effects of Posted Speed Limits on Speed Decisions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .17-8 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 C H A P T E R 17 Speed Perception, Speed Choice, and Speed Control 17-1

BEHAVIORAL FRAMEWORK FOR SPEEDING Introduction Behavioral framework for speeding refers to a conceptual overview of the key factors relevant to speed selection, as well as their relationship to potential speeding countermeasures. The figure below provides such a framework and attempts to capture the relevant driver, vehicle, roadway, and environment (DVRE) factors and to link these “predictor variables” to specific indices of driver behavior and driver performance. The factors and relationships depicted in the figure are firmly grounded in relevant studies and analyses of driver behavior. Specifically, it reflects past analyses and syntheses of the research literature on driver behavior and crash risk (1, 2), recent run-off-road safety work (3), safety countermeasures (4, 5), results from the recent 100-car study conducted by VTTI (6), as well as research that covers driving or crashes more generally (e.g., 7, 8, 9). Importantly, the framework includes a variety of countermeasure types, explicitly targeted at specific DVRE interactions. Design Guidelines Countermeasures intended to address speeding should consider the following representation of speed selection to fully understand the relative roles of situation, demographics, individual differences, and unexplained variance in predicting travel speeds. Driving Outcomes, including Excessive Speeding, Near-misses, and Crashes Countermeasure elements that address demographic or driver f itness aspects Countermeasure elements that address specif ic driver behaviors, attitudes, beliefs, etc. Countermeasure elements that address specif ic vehicle factors Countermeasure elements that target specif ic driver types Countermeasure elements that address specif ic situation/ environment factors Countermeasure elements that address specif ic driving actions Vehicle Factors - Type - Size - Age - Field of View - Handling - Antilock Braking Driver Performance - Speed Selection - Headway Selection - Lane Maintenance - Lane Change Behaviors - Illegal Actions Driver Profiles - Impaired - Reckless - Aggressive - “Normal” - Cautious - Capacity-limited - Distracted This characteristic changes that behavior Roadway Factors (Edge Marking, Lane Width, Signing, etc.) Intersection Factors (Signalized, Conf iguration, Signal Timing etc.) Environmental Factors (Lighting, Weather) Environment FactorsBackground Characteristics (Experience, Training, etc.) Demographic Characteristics (Age, Sex, etc.) Physiological Factors (Vision, Hearing, Health, Performance (RT), Sleep Patterns) Attitudes, Beliefs, Motivations, etc. - Attitudes toward Act - Critical Events (e.g., crashes) - Social Norms - Perceived Control - Habit - Intentions - Trip Types Driver Factors Traffic Factors (Speed, Volume) Based Primarily on Expert Judgment Based Equally on Expert Judgment and Empirical Data Based Primarily on Empirical Data HFG SPEED PERCEPTION, CHOICE, AND CONTROL Version 2.0 17-2

Discussion A substantial amount of research has been done on the causes of speeding and it is clear that speeding is a complex driving behavior. There is typically no single simple solution for addressing speeding concerns. The table below shows the multitude of factors that have been found to be associated with speeding or speed-related crashes. Despite all this research, there is still uncertainty regarding the relative importance of these factors and how this information can be used to develop countermeasures that effectively target specific types of drivers. The figure shown as part of the guideline on the previous page depicts how several of these factors’ corresponding countermeasures are related. FACTORS FOUND TO BE ASSOCIATED WITH SPEEDING IN PREVIOUS RESEARCH Factor Example Variables Example References (see Chapter 23 for full citations) Demographic Age, gender, socioeconomic and education level DePelsmacker & Janssens, 2007; Harré, Field & Kirkwood, 1996; Hemenway & Solnick, 1993; Stradling, Meadows & Beatty, 2002 Personality Attitudes, habits, personal and social norms, thrill-seeking, beliefs Arnett, Offer & Fine, 1997; Clément & Jonah, 1984; DePelsmacker & Janssens, 2007; Ekos Research Associates, 2007; Gabany, Plummer & Grigg, 1997; McKenna & Horswill, 2006; Stradling, Meadows & Beatty, 2002 Roadway Posted speed Book & Smigielski, 1999; Giles, 2004 Environment Urban/rural Giles, 2004; Rakauskas, Ward, Gerberich & Alexander, 2007 Vehicle Engine size; vehicle age Hirsh, 1986; Stradling, Meadows & Beatty, 2002 Risky Behaviors Drinking and driving, seatbelt use, red light running Arnett, Offer & Fine, 1997; Cooper, 1997; Gabany, Plummer & Grigg, 1997; Harré, Field & Kirkwood, 1996; Hemenway & Solnick, 1993; Rajalin, 1994 Situational Trip time, mood, inattention, fatigue Arnett, Offer & Fine, 1997; Ekos Research Associates, 2007; Gabany, Plummer & Grigg, 1997; Hirsh, 1986; McKenna, 2005; McKenna & Horswill, 2006 Design Issues None. 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. Campbell, J.L., Richard, C.M., Brown, J L., Nakata, A., and Kludt, K. (2003). Technical Synthesis of IVI Human Factors Research: Compendium of IVI Human Factors Research. Seattle, WA: Battelle Human Factors Transportation Center. 2. Kludt, K., Brown, J.L., Richman, J., and Campbell, J.L. (2006). Human Factors Literature Reviews on Intersections, Speed Management, Pedestrians and Bicyclists, and Visibility (FHWA-HRT-06-034). Washington, DC: FHWA. 3. LeBlanc, D., Sayer, J., Winkler, C., Ervin, R., Bogard, S., Devonshire, J., et al. (2006). Road Departure Crash Warning Field Operational Test. Washington, DC: NHTSA. 4. Various (2005). NCHRP Report 500: Guidance for Implementation of the AASHTO Strategic Highway Safety Plan, Volumes 1-17. Washington, DC: Transportation Research Board. 5. NHTSA (2007). Countermeasures that Work: A Highway Safety Countermeasure Guide for State Highway Safety Offices. Washington, DC. 6. Klauer, S.G., Dingus, T.A., Neale, V.L., Sudweeks, J.D., and Ramsey, D.J. (2006). The Impact of Driver Inattention on Near-Crash/Crash Risk: An Analysis Using the 100-Car Naturalistic Driving Study Data (DOT HS 810 594). Washington, DC: NHTSA. 7. Hendricks, D.L., Fell, J.C., and Freedman, M. (1999). The Relative Frequency of Unsafe Driving Acts in Serious Traffic Crashes, Summary Technical Report. Washington, DC: NHTSA. 8. Groeger, J.A. (2000). Understanding Driving: Applying Cognitive Psychology to a Complex Everyday Task. Hove, U.K.: Psychology Press. 9. Treat, J.R., Tumbas, N.S., McDonald, S.T., Shinar, R.D., Mayer, R.E., Sansifer, R.L., et al. (1979). Tri-level Study of the Causes of Traffic Accidents, Executive Summary (DOT HS 805 099). Bloomington: Indiana University. 17-3 HFG SPEED PERCEPTION, CHOICE, AND CONTROL Version 2.0

SPEED PERCEPTION AND DRIVING SPEED Introduction Speed perception refers to a driver’s judgment of how fast he or she is traveling. While direct speed information is available from the speedometer, drivers still rely heavily on cues from the environment to judge how fast they are traveling. Auditory (engine noise) and tactile (vibrations) information can influence speed perception; however, drivers’ primary basis for estimating their speed is the visual sensation provided by the highway geometrics and other information about objects in their immediate environment streaming through their visual field. If drivers underestimate their travel speed, they are traveling faster than they expect, and if they overestimate their travel speed, they will travel slower than they expect. Design Guidelines The driver’s perceptual experience of the roadway should be consistent with intended travel speed. There should be some consistency between relevant roadway cues and posted speeds. FACTORS THAT AFFECT SPEED PERCEPTION Factors that May Cause Drivers to UNDERESTIMATE Their Travel Speed Factors that May Cause Drivers to OVERESTIMATE Their Travel Speed Higher design standard Greater roadway width Divided, walled urban roads Rural roads without roadside trees Daylight compared to nighttime illumination conditions Two-lane narrow urban roads Roads densely lined with trees Transverse pavement markings GRAPHICAL EXAMPLE OF OPTIC FLOW FROM A CENTRAL FOCAL POINT, WHICH IS INDICATED AS THE RED BOX (FROM CNS VISION LAB (1)) Based Primarily on Expert Judgment Based Equally on Expert Judgment and Empirical Data Based Primarily on Empirical Data HFG SPEED PERCEPTION, CHOICE, AND CONTROL Version 2.0 17-4

Discussion In Fildes, Fletcher, and Corrigan (2), subjects viewed film presentation of moving scenes in a laboratory setting. The study was conducted to develop a suitable means of assessing the sensory perception of speed on the road and to evaluate the effects of several road and roadside features on the speed judgments of drivers. Among other findings, the researchers reported that drivers underestimated their travel speeds on roads with higher design standards, on roads with a greater width, on divided and wide urban roads, and on rural roads without roadside trees (compared to those with many trees). They tended to overestimate their speeds on two-lane narrow urban roads. In Triggs and Berenyi (3), subjects estimated speed under day and night conditions as passengers driving in a car on an unlit freeway. Speed was underestimated in both day and night conditions; however, judgments were more accurate at night than during the day. Importantly, centerline pavement-mounted reflectors provided a highly visible feature that was unavailable during the day. From three types of speed estimation—(1) a driver’s estimate of his/her own vehicle speed, (2) the estimation of approaching vehicle speed, and (3) detection of relative velocity when car-following—Triggs (4), a broad review of speed estimation studies, shows the following trends: Speed perception increases when transverse stripes are painted across the road with their separation progressively decreasing (though they may be effective only for drivers who are unfamiliar with the site). Speed judgments tend to be higher when a rural road is lined with trees. Speed judgments tend to be higher in low light conditions. During car-following, judgments of relative speed tend to be made more accurately when the gap between the two vehicles is closing rather than when it is opening. When car-following, observers in the following car tend to underestimate the relative speed difference between their car and the one in front of it. The figure on the previous page illustrates two important sources of information that underlie drivers’ speed perception. The first is the point of expansion, which is denoted by the red square, and the second is the optic flow, which is shown as the blue arrows. During forward motion, the point of expansion indicates the observers’ destination and appears stationary relative to the observer. All other points are seen as moving away from the point of expansion, and the relative motion of the optic flow points forms the basis for speed perception. Points that are closer to the observer appear to move faster than points closer to the point of expansion. Stronger and more consistent optic-flow cues (e.g., dense/cluttered visual environments, salient pavement marking, etc.) can amplify the sensation of speed through the environment and cause higher speed judgments. Design Issues Speed adaptation, which occurs for drivers who continue at a constant speed for an extended period of time, leads to drivers generally underestimating their speed in latter sections of extended tangent sections (4). This adaptation effect has implications for design elements requiring speed changes, such as horizontal curves, because drivers may be traveling faster than expected. Additionally, this effect may also carry over to nearby roadways (5). Milosevic and Milic (6) investigated the accuracy of speed estimation in sharp curves and the effect of advisory signs on speed estimation and found that drivers with over 11 years of experience significantly underestimated their speeds. Cross References Behavioral Framework for Speeding, 17-2 Effects of Roadway Factors on Speed, 17-6 Effects of Posted Speed Limits on Speed Decisions, 17-8 Key References 1. CNS Vision Lab (n.d.) Heading Perception: Where Am I Going? Retrieved November 24, 2009 from http://cns.bu.edu/visionlab/projects/buk/ 2. Fildes, B.N., Fletcher, M.R., and Corrigan, J.M. (1987). Speed Perception 1: Drivers' Judgements of Safety and Speed on Urban and Rural Straight Roads. (CR 54). Canberra, Australia: Federal Office of Road Safety. 3. Triggs, T.J., and Berenyi, J.S. (1982). Estimation of automobile speed under day and night conditions. Human Factors, 24(1), 111-114. 4. Triggs, T.J. (1986). Speed estimation. In G.A. Peters & B.J. Peters (Eds.), Automotive Engineering and Litigation (pp. 95-124). New York: Garland Press. 5. Casey, S.M., and Lund, A.K. (1987). Three field studies of driver speed adaptation. Human Factors, 29(5), 541-550. 6. Milosevic, S., and Milic, J. (1990). Speed perception in road curves. Journal of Safety Research, 21(1), 19-23. 17-5 HFG SPEED PERCEPTION, CHOICE, AND CONTROL Version 2.0

EFFECTS OF ROADWAY FACTORS ON SPEED Introduction The effects of roadway factors on speed refers to the impact of geometric, environmental, and traffic factors on driving speed under free-flow conditions in tangent roadway sections. Speed in curve entry is covered in Chapter 6. Free-flowing speed is defined as conditions in which a driver has the ability to choose a speed of travel without undue influence from other traffic, conspicuous police presence, or environmental factors. In other words, the driver of a free-flowing vehicle chooses a speed that he or she finds comfortable on the basis of the appearance of the road. Typically this involves a minimum headway time of 4 to 6 s (1). Note that although posted speed is often found to be one of the factors that is most strongly correlated with free-flow speed, this correlation is somewhat misleading, because driver compliance with posted speed can be low if the posted speed is set too low (see the guideline “Effects of Posted Speed Limits on Speed Decisions,” on page 17-8). In contrast, the strong association between posted speed and free-flow speed typically occurs because the 85th percentile speed is often used to set the posted speed limit. Design Guidelines The following factors that appear to be associated with drivers’ choosing a higher travel speed should be considered when designing roadways. Strength of Empirical Evidence Factors Associated with HIGHER Free-Flow Speeds Rural Highways Low-Speed Urban Streets Higher Design Speed Solid Solid Grade Solid Solid Wider Lane Width — Mixed Higher Access Density Solid Mixed Separated Bicycle Lanes — Mixed Less Pedestrian/Bicycle Side Friction — Mixed No Roadside Parking — Mixed Number of Lanes Solid — Shoulder Width Mixed — Based Primarily on Expert Judgment Based Equally on Expert Judgment and Empirical Data Based Primarily on Empirical Data HFG SPEED PERCEPTION, CHOICE, AND CONTROL Version 2.0 17-6

Discussion As the table on the previous page makes clear, the empirical record is far from conclusive with respect to the ability to predict drivers’ speed choices associated with relevant geometric, environmental, and traffic factors. Nonetheless, some relationships between drivers’ speed choices and these factors—however tentative—have emerged from the literature and are worth presenting here. Currently, there is insufficient research to provide more quantitative guidance about how much the factors listed in the guideline table increase free-flow speed. As seen in the figure below, roadway factors impact both the driver’s choice of speed, as well as overall crash probability and severity. In Fitzpatrick, Carlson, Brewer, and Wooldridge (3), data were collected at 24 horizontal curve sites and 36 straight section sites to identify roadway factors that influence speed. Data collected included details of alignment (e.g., curve radius, curve length, straight section length), cross section (e.g., lane width, superelevation, median characteristics), roadside details (e.g., access, density, pedestrian activity), and information on traffic control devices. Laser guns were used to collect speed from vehicles at the 60 (total) sites. Multiple regression techniques, using 85th percentile speed as a “quantifiable definition of operating speed,” were used in the analysis. The alignment (downstream distance to control) and cross section (lane width) factors explained about 25% of the variability in the speed data for both curve and straight road sections. Roadside factors were not significant for the straight road sections, but accounted for about 40% of the variability in the speed data for curves. Additional analyses conducted without using posted speed limits resulted in only lane width as a significant variable for straight road sections, with both median presence and roadside development as significant variables for curves. Source: Milliken, J.G., Council, F.M., Gainer, T.W., Garber, N.J., Gebbie, K.M., Hall, J.W., et al. (2) Design Issues None. Cross References Design Consistency in Rural Driving, 16-8 Behavioral Framework for Speeding, 17-2 Key References 1. Parker, M.R., Jr. (1997). Effects of Raising and Lowering Speed Limits on Selected Roadway Sections. (FHWA-RD-92-084). McLean, VA: FHWA. 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. Fitzpatrick, K., Carlson, P., Brewer, M., and Wooldridge, M. (2001). Design factors that affect driver speed on suburban streets. Transportation Research Record, 1751, 18-25. 17-7 HFG SPEED PERCEPTION, CHOICE, AND CONTROL Version 2.0

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). 0 10 20 30 40 23 28 33 43 48 53 58 63 68 73 78 83 88 MPH JAN, FEB, MAR 1995 1996 0 10 20 30 40 23 28 33 43 48 53 58 63 68 73 78 83 88 MPH APR, MAY, JUN 1995 1996 0 10 20 30 40 23 28 33 43 48 53 58 63 68 73 78 83 88 P E R C E N T O F D R IV E R S P E R C E N T O F D R IV E R S P E R C E N T O F D R IV E R S MPH JUL, AUG, SEP 1995 1996 Source: recreated from Milliken et al. (2) Based Primarily on Expert Judgment Based Equally on Expert Judgment and Empirical Data Based Primarily on Empirical Data HFG SPEED PERCEPTION, CHOICE, AND CONTROL Version 2.0 17-8

Discussion It is quite clear from both everyday observation and existing research data that mo st 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 co mp ly with posted speed limits, specifically (1) 40.8% exceeded posted speed lim its 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 governm ents on appropriate methods of setting speeds limits and related enforcement strategies. Wi th respect to driver perceptions of speeding and speed limits, the review found that (1) most drivers do not percei ve speeding as a particularly risky activity; (2) mo st dr iv ers 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 co mp liance gradually eroded. Drivers violate new, higher speed lim its because they expect the sam e enforcement tolerance of 5 to 10 mi/h at the higher limits. Specifically, average and 85 th percentile speed typically increased 1 to 3 m i/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 num ber 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 enforcem ent is the mo st comm on mediator between speed limit and speed choice. Where speed choice is not constrained by speed limits and their enforcem ent, 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 doe s 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 highe r speeds once they switch to a connecting roadway. The evidence for carryover effects is lim ite d, especially because many studies fi nd such a sm all relationship between posted speed limit change and free-flow speed on the principal roads ( 4, 2 ). Another issue that may be worth exam ining 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 Counterm easures: Setting Appropriate Speed Limits, 17-10 Speeding Counterm easures: Comm unicating Appropriate Speed Lim its, 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 mu ltivariate analysis. Transport Policy, 11 (3), 227-235. 2. Milliken, J.G., Co uncil, F.M., Gainer, T.W., Garber, N.J., Gebbie, K.M., Hall, J.W., et al. (1 998). Special Report 254: Managing Speed: Review of Current Practice for Setting and Enforcing Speed Limits . Wash ington, DC: Transportation Research Board. 3. Harkey, D.L., Robertson, H.D., and Davis, S.E. (1990). Assess me nt 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 m ph speed li m it. Transportation Quarterly, 44 (4), 533-547. 17-9 HFG SPEED PERCEPTION, CHOICE, AND CONTROL Version 2.0

SPEEDING COUNTERMEASURES: SETTING APPROPRIATE SPEED LIMITS Introduction Setting appropriate speed limits refers to guidelines and best practices for determining appropriate speed limits that take into account the unique traffic, design, and environmental aspects of the roadway. Much of the information in this guideline, as well as its companion guidelines (“Speeding Countermeasures: Communicating Appropriate Speed Limits” on page 17-12 and “Speeding Countermeasures: Using Roadway Design and Traffic Control Elements to Address Speeding Problems,” on page 17-14), are adapted from Neuman et al. (1). As part of NCHRP Report 500: Guidance for Implementation of the AASHTO Strategic Highway Safety Plan, the study by Neuman et al. (1) was developed to address two key problems involved in excessive or inappropriate speeds: (1) driver behavior (i.e., deliberately driving at an inappropriate or unsafe speed) and (2) driver response to the roadway environment (i.e., inadvertently driving at an inappropriate or unsafe speed, failure to change speed in a proper or timely manner, or failure to perceive the speed environment). Both these problems result in an increased risk of a crash or conflict. Design Guidelines The design guidelines below should be used to help set appropriate speed limits. Additional guideline information is provided in the discussion section; however, the original source of these recommendations—Neuman et al. (1)—should be consulted for more specific design guidance. Objective General Strategy Design Guideline Set speed limits that account for roadway design, as well as traffic and environmental conditions Consider the: Design speed of a major portion of the road, Vehicle operating speed, measured as a range of 85th percentile speeds taken from spot speed surveys of free-flowing vehicles on the roadway, Safety experience of the roadway, in the form of crash frequencies and outcomes, and Enforcement experience; i.e., law enforcement’s allowance for driving above the posted speed limit as well as the level of enforcement. Implement variable speed limits (VSLs) While the efficacy of VSLs is uncertain (see also Milliken et al. (2)), they can be used for: Predictable events, such as during school hours and construction activities, and Unpredictable events, such as poor visibility due to fog or snow, and traffic incidents. Set appropriate speed limits Implement differential speed limits for heavy vehicles (high-speed areas only) In high-speed areas, consider posting a lower speed limit for heavy trucks in order to reduce the severity of collisions involving trucks. Note: Not all researchers agree that differential speed limits for trucks should be used, see the following discussion section. Based Primarily on Expert Judgment Based Equally on Expert Judgment and Empirical Data Based Primarily on Empirical Data HFG SPEED PERCEPTION, CHOICE, AND CONTROL Version 2.0 17-10

Discussion As discussed in Neuman et al. (1), speed limits that appear inconsistent, fail to reflect the immediate roadway environment, or are inconsistent with driver expectancies may be ignored by drivers. This situation, in turn, can contribute to a lack of respect for and compliance with speed limits. The posted speed limit provides drivers with not just a legal limit, but also the maximum speed that highway engineers and road designers consider to be safe and appropriate. As noted by Milliken et al. (2), well-conceived speed limits also provide the basis for enforcement by law enforcement officers and the court system. For the set speed limits that account for roadway design, as well as traffic and environmental conditions strategy, practicality and enforcement are key considerations. Setting the speed limit at the 85th percentile speed is expected to result in compliance by most drivers; however, unique design, traffic, or environmental characteristics of the roadway can also affect actual driving speeds. Such characteristics include proximity to schools or hospitals, an unusually high percentage of trucks in the traffic flow, unusually heavy pedestrian volumes, or a concentration of elderly pedestrians. Variable speed limits (VSLs) are generally communicated through CMSs or other traffic control devices. A critical issue with VSLs is determining where they should be used, when the speed limits should be changed, and what the “other” speed limits should be; cameras or other detection equipment can be used to make these determinations (1). Visible and regular enforcement is also required to ensure compliance with the speed limits. The use of differential speed limits for heavy trucks is an option for locations associated with a high incidence of truck crashes; however, the research is mixed with respect to the efficacy of doing so. The logic underlying the use of having a lower posted speed limit for trucks than for passenger vehicles is “that trucks have much longer stopping distances than do light vehicles and have other speed-related risks such as rollover at lower speeds and vulnerability to loss of control in cross winds” (3). The counterargument is that differential speed limits for trucks vs. cars increases the overall variability in vehicle speeds (at a given location at a given time), resulting in a greater potential for conflicts and crashes between trucks and cars. In a review of safety outcomes associated with heavy vehicles, Harwood, Potts, Torbic, and Glauz (4) found that the use of differential speed limits does not seem to reduce crashes, but may vary the distribution of crash types. Design Issues This guideline, and its companion guidelines (“Speeding Countermeasures: Communicating Appropriate Speed Limits” on page 17-12 and “Speeding Countermeasures: Using Roadway Design and Traffic Control Elements to Address Speeding Problems” on page 17-14), only include those countermeasures provided by Milliken et al. (2) that are directed at roadway design. Neuman et al. (1) should be consulted for a more detailed discussion of these countermeasures, as well as countermeasures intended (1) to heighten driver awareness of speeding-related safety issues and (2) to improve the efficiency and effectiveness of speed enforcement efforts. Cross References 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. Neuman, T.R., Slack, K.L., Hardy, K.K., Bond, V.L., Potts, I., and Lerner, N. (2009). NCHRP Report 500: Guidance for Implementation of the AASHTO Strategic Highway Safety Plan, Volume 23: A Guide for Reducing Speeding-Related Crashes. Washington, DC: Transportation Research Board. 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. Knipling, R.R., Waller, P., Peck, R.C., Pfefer, R., Neuman, T.R., Slack, K.L., et al. (2004). NCHRP Report 500: Guidance for Implementation of the AASHTO Strategic Highway Safety Plan, Volume 13: A Guide for Reducing Collisions Involving Heavy Trucks. Washington, DC: Transportation Research Board. 4. Harwood, D.W., Potts, I.B., Torbic, D.J., and Glauz, W.D. (2003). CTBSSP Synthesis of Safety Practice 3: Highway/Heavy Vehicle Interaction. Washington, DC: Transportation Research Board. 17-11 HFG SPEED PERCEPTION, CHOICE, AND CONTROL Version 2.0

SPEEDING COUNTERMEASURES: COMMUNICATING APPROPRIATE SPEED LIMITS Introduction Communicating appropriate speed limits refers to guidelines and best practices for communicating posted speed limits to drivers. Much of the information in this guideline, as well as its companion guidelines (“Speeding Countermeasures: Setting Appropriate Speed Limits” on page 17-10 and “Speeding Countermeasures: Using Roadway Design and Traffic Control Elements to Address Speeding Problems” on page 17-14), are adapted from Neuman et al. (1). As part of NCHRP Report 500: Guidance for Implementation of the AASHTO Strategic Highway Safety Plan, the study by Neuman et al. (1) was developed to address two key problems involved in excessive or inappropriate speeds: (1) driver behavior (i.e., deliberately driving at an inappropriate or unsafe speed) and (2) driver response to the roadway environment (i.e., inadvertently driving at an inappropriate or unsafe speed, failure to change speed in a proper or timely manner, or failure to perceive the speed environment). Both these problems result in an increased risk of a crash or conflict. Design Guidelines The design guidelines below should be used to help communicate appropriate speed limits. Additional guideline information is provided in the discussion section; however, the original source of these recommendations— Neuman et al. (1)—should be consulted for more specific design guidance. Objective General Strategy Design Guideline Improve speed limit signage Locate speed limit signs where drivers expect them to be, such as following a major intersection. Use advance notice signs (e.g., “Reduced Speed Ahead”) to alert the driver to an upcoming speed change. Consider context: where other traffic signs and/or commercial signs are abundant, use larger speed signs, increase the number of speed signs, or remove unnecessary signs. Implement active speed warning signs Use in locations where speeding has been observed or poses a safety risk, such as school zones, sharp horizontal curves, or locations with a history of speed-related crashes. Use in-pavement measures to communicate the need to reduce speeds May include transverse lines, peripheral transverse lines, chevron lines, and rumble strips. Communicate appropriate speeds through the use of traffic control devices Implement changeable message signs (high-speed areas only) Use CMSs to present information relevant to traffic conditions, work zones, weather and road surface conditions, detour/directional information, crashes and incidents, and appropriate speed limits. Based Primarily on Expert Judgment Based Equally on Expert Judgment and Empirical Data Based Primarily on Empirical Data HFG SPEED PERCEPTION, CHOICE, AND CONTROL Version 2.0 17-12

Discussion As discussed in Neuman et al. (1), information about speed limits—in the form of signs or markers—should be clearly communicated to drivers, at appropriate locations on the roadway. The posted speed limit provides drivers with not just a legal limit, but also the maximum speed that highway engineers and road designers consider to be safe and appropriate. The placement and visibility of speed signs are key to properly communicating speed limits. Improving speed limit signage is especially important in areas where signs are frequently obscured by other signage, vegetation, or adverse weather conditions. Also, having a high percentage of older drivers on a particular section of the roadway is often a good reason to address signage location and visibility. Providing conspicuous and redundant information about unexpected posted speed changes, such as those greater than 10 mi/h, can also increase driver awareness of a speed change. This information can be provided by using “Speed Reduction Ahead” signs in advance of the change, placing signs on both sides of the roadway, and using signs with salient features (e.g., fluorescent flags) (1). Additional supplementary signs spaced every 60 s of travel (or more frequently in urban areas with increased access to the road) can also promote driver awareness of the speed limit. Active speed warning signs improve drivers’ awareness of both their current speed and the posted speed limit in order to deter speeding behaviors. In Bloch (2), a before–after evaluation was conducted to assess the benefits of using a speed warning sign. The study found that mean speed was reduced at the sign location, but that intermittent enforcement was required to significantly reduce speeds downstream from the sign. The sign was effective in reducing excessive speeds (i.e., speeds 10 mi/h above the posted speed). In-pavement measures and other perceptual measures can be used to encourage drivers to adhere to speed limits (1). Pavement marking—such as transverse lines, peripheral transverse lines, and chevron lines—gives the illusion that the driver is driving faster than his/her actual speed and can be used as a means to decrease excessive speeds by reducing the driver’s comfort level at higher speeds (1). These approaches can be used along any roadway segment where speed may be a problem, as well as locations where speed reductions are necessary, such as intersection approaches, work zones, toll plazas, and ramps. Rumble strips (e.g., continuous shoulder rumble strips, centerline rumble strips, or transverse rumble strips) may also be used to reduce vehicle speeds or to prevent crashes where speed is a causal factor (1). In this role, rumble strips are used as a traffic calming device in, for example, high- pedestrian areas such as parks, schools, hospitals, and residential areas. Rumble strips are also discussed in “Shoulder Rumble Strips” on page 16-6. CMSs can also be used to display information on appropriate speeds relative to current conditions. See Chapter 19 for more details on when and how to use CMSs. Design Issues This guideline, and its companion guidelines (“Speeding Countermeasures: Setting Appropriate Speed Limits” on page 17-10 and “Speeding Countermeasures: Using Roadway Design and Traffic Control Elements to Address Speeding Problems,” on page 17-14), only include those countermeasures provided by Milliken et al. (3) that are directed at roadway design. Neuman et al. (1) should be consulted for a more detailed discussion of these countermeasures, as well as countermeasures intended (1) to heighten driver awareness of speeding-related safety issues and (2) to improve the efficiency and effectiveness of speed enforcement efforts. Cross References Speeding Countermeasures: Setting Appropriate Speed Limits, 17-10 Speeding Countermeasures: Using Roadway Design and Traffic Control Elements to Address Speeding Problems, 17-14 Rumble Strips, 16-6 Key References 1. Neuman, T.R., Slack, K.L., Hardy, K.K., Bond, V.L., Potts, I., and Lerner, N. (2009). NCHRP Report 500: Guidance for Implementation of the AASHTO Strategic Highway Safety Plan, Volume 23: A Guide for Reducing Speeding-Related Crashes. Washington, DC: Transportation Research Board. 2. Bloch, S.A. (1998). A comparative study of the speed reduction effects of photo-radar and speed display boards. Transportation Research Record, 1640, 27–36. 3. 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. 17-13 HFG SPEED PERCEPTION, CHOICE, AND CONTROL Version 2.0

SPEEDING COUNTERMEASURES: USING ROADWAY DESIGN AND TRAFFIC CONTROL ELEMENTS TO ADDRESS SPEEDING PROBLEMS Introduction Using roadway design and traffic control elements to address speeding problems refers to guidelines and best practices for selecting and using geometric design features and traffic signals to support safe speed decisions by drivers. Much of the information in this guideline, as well as its companion guidelines (Speeding Countermeasures: Setting Appropriate Speed Limits on page 17-10 and Speeding Countermeasures: Communicating Appropriate Speed Limits on page 17-12), is adapted from Neuman et al. (1). As part of NCHRP Report 500: Guidance for Implementation of the AASHTO Strategic Highway Safety Plan, the study by Neuman et al. (1) was developed to address two key problems involved in excessive or inappropriate speeds: (1) driver behavior (i.e., deliberately driving at an inappropriate or unsafe speed) and (2) driver response to the roadway environment (i.e., inadvertently driving at an inappropriate or unsafe speed, failure to change speed in a proper or timely manner, or failure to perceive the speed environment). Both these problems result in an increased risk of a crash or conflict. Design Guidelines The design guidelines below should be used to select and use geometric design features and traffic signals to support safe speed decisions by drivers. Additional guideline information is provided in the discussion section; however, the original source of these recommendations—Neuman et al. (1)—should be consulted for more specific design guidance. Objective General Strategy Design Guideline Use consistent combinations of geometric elements to control speeds. Design features such as curve radius, tangent length, length of spirals, vertical grades and curves, available sight distance, and cross- section features should be designed consistently across locations, in a manner that meets driver expectancies. Provide adequate change + clearance intervals at signalized intersections. Clearance intervals should account for expected approach speeds and should reflect operating speeds, intersection width, vehicle lengths, and driver characteristics such as reaction time and braking. See Tutorial 4 for the equation developed by ITE (2) for determining clearance intervals. Provide protected left turns. Implement protected-only signal phasing for left turns at high-speed signalized intersections. Ensure that roadway design and traffic control elements support appropriate and safe speeds Provide improved visibility. Install lighting at high-speed sections of the roadway, especially intersections. Based Primarily on Expert Judgment Based Equally on Expert Judgment and Empirical Data Based Primarily on Empirical Data HFG SPEED PERCEPTION, CHOICE, AND CONTROL Version 2.0 17-14

Discussion As discussed in Neuman et al. (1), while drivers ultimately select their own speeds, they receive, process, and use a number of cues from the immediate driving environment when doing so. Key elements of the driving environment that can effectively communicate safe speeds are roadway design and the use and operation of traffic control devices. Design consistency is a key principle in roadway design. Using consistent combinations of geometric elements leads to roadway elements that meet driver expectancies and can result in consistent speeds and fewer unexpected speed changes. For example, large differences and sudden changes in horizontal alignment, available sight distance, curve radii, etc. should be avoided, as these can increase driver workload, misperceptions, errors, and—ultimately—the likelihood of crashes. Clearance intervals provide safe transitions in right-of-way (ROW) assignment between crossing or conflicting flows of traffic. One way to accomplish safe transitions is an all-red interval, which should be designed to account for expected approach speeds to reduce the likelihood of collisions resulting from red light running. Clearance intervals that are too short can result in drivers not being able to stop in time for the red light; drivers can also stop too quickly, increasing the risk of rear-end collisions from following vehicles. Clearance intervals that are too long can lead to driver impatience, or red light running, especially in drivers familiar with the intersection. Whether the concern is red light running or increased risk of collisions, both outcomes are exacerbated by speeding. On high-speed roadways, especially in high traffic volume situations, there may be inadequate gaps for left-turning vehicles. Protected-only left-turn signals have a phase designated specifically for left-turning vehicles. Other factors that may warrant the use of protected-only left-turn phases include delay, visibility, distance of the intersection, and safety at the intersection (e.g., crash history) (1). The benefits of protected-only left turns include increasing left-turn capacity and reducing intersection delays for vehicles turning left (3). The use of protected left- turn phases also improves safety by removing conflicts during a left-turn movement. This improved safety can be especially important on roadways where high operating speeds can contribute to the crash severity and may play a role in the difficulty a driver has with identifying and selecting a safe gap (1). However, the use of protected-only left-turn signals will usually increase the cycle length, which also increases delay. For additional discussion and guidance on the type of left-turn phase to use in a given situation, see Pline (4). On high-speed roads, drivers have less time to detect visual information because vehicles are traveling faster. This problem is compounded at night when the visual contrast of some roadway elements is reduced and drivers require more time to detect visual information (drivers at higher speeds will also travel farther during this elongated detection period and consequently have less time to react to hazards). While increasing lighting on its own will not prevent speeding, it will make potential hazards or other important information easier for drivers to see, particularly during nighttime and adverse weather conditions. Design Issue This guideline, and its companion guidelines (“Speeding Countermeasures: Setting Appropriate Speed Limits” and “Speeding Countermeasures: Communicating Appropriate Speed Limits”), only include those countermeasures provided by ITE (2) that are directed at roadway design. Neuman et al. (1) should be consulted for a more detailed discussion of these countermeasures, as well as countermeasures intended (1) to heighten driver awareness of speeding-related safety issues and (2) to improve the efficiency and effectiveness of speed enforcement efforts. Cross References Speeding Countermeasures: Setting Appropriate Speed Limits, 17-10 Speeding Countermeasures: Communicating Appropriate Speed Limits, 17-12 Design Consistency in Rural Driving, 16-8 Key References 1. Neuman, T.R., Slack, K.L., Hardy, K.K., Bond, V.L., Potts, I., and Lerner, N. (2009). NCHRP Report 500: Guidance for Implementation of the AASHTO Strategic Highway Safety Plan, Volume 23: A Guide for Reducing Speeding-Related Crashes. Washington, DC: Transportation Research Board. 2. ITE (1994). Determining Vehicle Signal Change and Clearance Intervals. Washington, DC. 3. Brehmer, C.L., Kacir, K.C., Noyce, D.A., and Manser, M.P. (2003). NCHRP Report 493: Evaluation of Traffic Signal Displays for Protected/Permissive Left-Turn Control. Washington, DC: Transportation Research Board. 4. Pline, J.L. (1996). NCHRP Synthesis of Highway Practice 225: Left-Turn Treatments at Intersections. Washington, DC: Transportation Research Board. 17-15 HFG SPEED PERCEPTION, CHOICE, AND CONTROL Version 2.0

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Human Factors Guidelines for Road Systems: Second Edition Get This Book
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TRB’s National Cooperative Highway Research Program (NCHRP) Report 600: Human Factors Guidelines for Road Systems: Second Edition provides data and insights of the extent to which road users’ needs, capabilities, and limitations are influenced by the effects of age, visual demands, cognition, and influence of expectancies.

NCHRP Report 600 provides guidance for roadway location elements and traffic engineering elements. The report also provides tutorials on special design topics, an index, and a glossary of technical terms.

The second edition of NCHRP 600 completes and updates the first edition, which was published previously in three collections.

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