Managing Speed Review of Current Practices for Setting and Enforcing Speed Limits -- Special Report 254 (1998) / Chapter Skim
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Appendix C: Effect of Speed Limits on Speed Distributions and Highway Safety
Appendix C: Effect of Speed Limits on Speed Distributions and Highway Safety
Pages 277-358
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From page 277... ...
In general, the proposition that changes in speed limits induce observed changes in the distribution of speeds on a road network and have implications for the network's highway safety is straightforward. However, testing this proposition 277
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In combination with differential speed limits, traffic enforcement, and other government interventions, the nation's highways produce trips and, as a by-product, highway safety outcomes. If all roads and all road users were homogeneous, then determining the effect of alternative speed limit policies on speed distributions and highway safety would be relatively easy.
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To date, little scientific information is available on the effect this has had on speed distributions and highway safety. By examining the empirical relationships among speed limits, speed distributions, and highway safety on nonlimited- and limitedaccess roads, this paper complements recent international reviews of speed limits and highway safety (Fildes and Lee 1993; Knowles et al.
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The known information on the effects of changing posted speed limits on the distribution of speeds is discussed. The effect of speed distributions on highway safety is examined.
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Although analysts can observe posted speed limits, measure the distribution of speeds on roadways, and observe safety outcomes, analysts cannot directly observe a driver's optimal speed. However, analysts can potentially (i.e., under certain conditions)
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A related question concerns the effect of changes in the posted speed limit on speed distributions and driver compliance with the posted speed limit. A second group of empirical studies examines the relationship between attributes of the speed distribution, specifically average speed and speed dispersion, and highway safety.
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Part of the current debate in this area concerns how the law affected alternative highway safety measures. METHODOLOGICAL CONSIDERATIONS Empirical studies examining the effect of changes in posted speed limits on speed distributions or highway safety generally identify the primary null hypothesis as either (or at times both)
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From page 284... ...
Similarly, numerous hypotheses concerning the effect of relaxed speed limits on highway safety outcomes are tested. As with highway speed distributions, these hypotheses often focus on direct and indirect effects, as well as the effects by vehicle type, road type, time of travel, location, alcohol consumption, and socioeconomic factors (e.g., age and gender)
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After controlling for confounding factors that reflect heterogeneity across roads and road users, the analyst tests whether there are statistically significant differences between speed distributions and highway safety outcomes on the affected and unaffected roads. Depending on whether the null hypothesis is accepted, the analyst concludes that the change in speed limit has or has not had an effect.
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rural Interstate states n21 n22 where nij (i, j = 1, 2) is the number of fatal crashes in state type i and speed limit environment j.
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In a time series regression model, N = 1 and there is a single cross section (e.g., annual nationwide fatal crashes from 1970 through 1997, or the monthly fatality rate from January
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Consider, for example, a regression analysis of fatalities on 50 observations, where each observation represents the number of fatalities in a state during 1990. Because not all states with eligible mileage increased their rural Interstate speed limits when Congress passed STURA, the sample of observations includes states with 65-mph (105-km/h)
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in the experimental states have a significant effect on the overall mean relative to the comparison states. Interrupted Time Series Analysis A third methodology often used to analyze the effect of altered speed limits on speed distributions and highway safety is time series intervention models.
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Essentially, the assumption is made that the disruption occurring in the series is due only to the policy under study. For example, consider the effect that relaxed rural Interstate speed limits have on fatalities.
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Studies that have addressed the effects of changing posted speed limits on speed distributions can be usefully divided by two road types, nonlimited-access and limited-access roads. Nonlimited-Access Roads Table C-1 identifies recent studies that have examined the speed distribution effects of posted speed limits on nonlimited-access roads.
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Casey and Experimental/compar- ANOVA Increase in average speeds Retested road configura Lund ison site data Multiple regression with no change in speed tions from 1987 study 1992 Reanalysis of 1985 CA limits Lower speeds for commer study Continued support for speed cial vehicles 1988 adaptation Sronger evidence that No increase in adapted younger drivers and speeds female drivers travel faster What is the geographical extent of adaptation?
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From page 293... ...
Ullman Six urban fringe 55- Before/after analysis Lowering speed limits from No control for confound mphb sites in Texas 55 b to 45 mphb had little and ing factors Dudek effect on speed distribu 1987a tions Parker Experimental/compar- Quasi-experimental Increased speed limits have 100 experimental sites; 83 1997a ison site data, 22 significant but small comparison sites states absolute effect on speeds Nonrandom site selection June 1986July 1988 Change in speed alone has No control for cross-site Aug. 1987July 1989 little effect on driver and cross-state differ behavior ences (e.g.
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It was further concluded that changes in posted speed limits had little effect on highway safety. The latter result is expected given the finding that altered limits had little effect on speed distributions.
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of experimental site roads included in the analysis. · Since, for all experimental sites included in the study, speed limits were to be changed, the posted speed limit changes may have simply rationalized observed behavior.
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However, there was no control for other confounding factors, including population changes, traffic congestion, and traffic enforcement, which weakens the authors' conclusions that lowering speed limits below the 85th percentile speed had no "conclusive effect on absolute speeds, speed distributions, or speed-changing activities" (Ullman and Dudek 1987, 48)
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speed limit that California implemented on a portion of its eligible roads. Although none of the sites included in the study were eligible for the higher speed limit, the authors found that average speed increased at two of the three freeway sites and three of the four connecting roads in the study.
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Similar to most studies in the highway safety literature, the studies cited in Table C-2 use a quasi-experimental design methodology, and none sufficiently controls for other factors that may influence speed distributions. Although one cannot conclude from the existing evidence that, all else constant, an increase in rural Interstate speed limits caused an increase in mean and 85th percentile speeds, the immediate and persistent increase in speeds identified in this work is
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effect 4th qtr. 19851st of speed limit on average speed qtr.
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b 55-mphb roads tics 1995 85th percentile speed = 64.0,b 1993 range = (56.4, 68.3) b 6.5 million citations, range = (2,081, 980,258)
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3.9-mph b increase in rural Mace and 51 rural Interstate Before/after analy- No control for confounding Heckard speed study sites sis Interstate speeds effects 4.3-mph b increase in rural 1991 in AL, AZ, CA, For spillover analysis, FL, IL, OH, TN, Interstate 85th percentile speed "toward/away" approach used 0.65-mph b increase in rural TX for some states 1986, 1988/1989 Interstate speed dispersion Small number of spillover sites Little change in speeds from 1988 Detailed analysis of Illinois data to 1989 Dual speed limits inhibit car Lttle local spillover effect observed speeds and no evidence of spillover onto urban Interstates Average car speed in 65-mphb states Freedman Northeastern states Quasi-experimental Increased rural Interstate speeds in 65-mph b states and (CT, MD, MA, less than in dual speed limit states Williams NJ, NY, PA, NH, No effect on rural Interstate speeds No statistical tests in 55-mph b states 1992 OH, VT, VA, No control for differences across WV) Lower truck speeds in dual limit states or systems Oct.
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b VA) Changes in average speed and April 1989August 85th percentile speed less than 1989 0.5 mphb at comparison sites a The text covers this study in more detail.
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On the other hand, the mixed evidence on speed distributions in 55-mph (89-km/h) environments, combined with a lack of control for other confounding factors, precludes the drawing of any firm conclusions from these studies concerning the spillover effects of relaxed rural Interstate speed limits.
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They also imply that conclusions based on aggregates may be oversimplified and that aggregation masks the underlying mechanisms through which speed limit changes affect speed distributions. The analysis by McKnight et al.
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SPEED DISTRIBUTION AND HIGHWAY SAFETY Studies The preceding section summarized a set of empirical studies examining the effect of changes in speed limits on speed distributions on nonlimited-access and limited-access roads. Particularly for relaxed rural Interstate speed limits passed in 1987, there is evidence, albeit not consistent, that relaxed speed limits on rural Interstate highways increased mean and 85th percentile speeds as well as speed dispersion on these roads.
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Recent work, identified in Table C-3, has typically centered on the highway safety effects of average speeds versus speed dispersion. In contrast, as will be seen in the next section, a significant amount of research exists on the effect of speed limits on highway safety.
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is U speed shaped McCarthy Countywide Regression analysis After controlling for speed dis- Controls for a variety of 1988 State of Indiana, persion, average speed had lit- socioeconomic factors 1987 tle effect on safety Limited sample (continued on next page)
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Table C-3 (continued) Study Database for Speeds Methodology Major Findings Comments Levy and Statewide data for Regression analysis Average speed, speed dispersion, Controls for a variety of Asch 1985 and their interaction have socioeconomic factors 1989 important effects on the fatal- Speed dispersion significantly ity rate increased Interstate fatality rates when average speed 63.8 mphb Average speed significantly increased Interstate fatality rates when speed dispersion 8.2 mphb Similar results for total fatal ity rates Significant speed effect results from weighted average speed measures (Lave 1989)
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Fowles and Statewide data for Regression analysis Speed and speed dispersion con- Results robust to model speci Loeb 1979 sistently significant determi- fication 1989 nants of fatalities Controls for a variety of socioeconomic factors Uses Interstate speeds to explain total statewide fatalities (Lave 1989) Snyder Primary federal-aid Regression analysis Average speed and speed disper- Lack of control for confound 1989 rural highways sion for the fastest vehicles are ing effects offset, to some Annual data: significant determinants of degree, by fixed effects 19721974 for fatalities model 26 states Speed dispersion for the slowest Rural speed data reflect vehicles is unimportant Interstates, rural primary and secondary roads (Lave 1989)
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From page 310... ...
Lave did not use these findings to support higher posted speed limits because there is little information on the relationship between speed dispersion and average speed. For his data, there was "generally a negative correlation" between the two, but he took this only as suggestive of a negative relationship.
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Among the findings from these studies are the following: · After controlling for other influences on safety, average speed and speed dispersion are both important determinants of highway safety.
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As noted by Lave, there is a need to obtain improved data on a road's speed characteristics as well as associated data that control for other determining factors of highway safety. This would reduce potential aggregation problems and enable researchers to better isolate the effects of average speed and speed dispersion on highway safety.
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From page 313... ...
Minimum speed limits could also reduce speed dispersion, but more research is needed to evaluate the effects of minimum speed limits on speed distribution and highway safety. · More detailed and informative data should be collected to better understand the relationship between average speed, speed dispersion, and highway safety.
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RELAXED RURAL INTERSTATE SPEED LIMITS AND HIGHWAY SAFETY Preceding sections have reviewed the empirical literature on the effects of posted speed limits on speed distributions and work on how changes in speed distributions affect highway safety. As shown in Figure C-1, the extent to which changes in posted speed limits affect highway safety depends on their effects on drivers' optimal speeds and, therefore, on the distribution of speeds.
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and Highway Safety -- National Perspective Table C-4 identifies 14 studies analyzing the overall effects of relaxed rural Interstate (RI) speed limits on highway safety.
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Table C-4 Research on 65-mpha Speed Limit and Highway Safety -- U.S. National Perspective Study Database for Study Methodology Major Findings Comments 65-mpha states NHTSA Thirty-eight 65- Before/after Inconclusive results on selective 1989ab mpha states Regression analysis 19% increase in RI fatalities, speed limit increases and Ten 55-mpha states with comparison 19861987 dual speed limits Annual data: series 7% decrease in UI fatalities, Limited control for confound 19751987 19861987 ing factors 55-mpha states Small numbers problem for 7% increase in RI fatalities, some state analysis 55-mpha states -- East Coast 19861987 10% increase in UI fatalities, Limited evidence of increases 19861987 in nonfatal injuries Before/after analysis 65-mpha states NHTSA Thirty-eight 65- Updates 1989 study mpha states 1990 Regression analysis 13% increase in RI fatalities, Limited control for confound Ten 55-mpha states with comparison 19871988; 2% decrease, ing factors Annual data: series 19881989 Variability across states 19751988 7% increase in RI fatality rate, 20% increase in RI vehicle 19871988; 7% decrease, miles traveled, 19861989, 19881989 and accounts for one-third of 55-mpha states increase in fatalities 12% decrease in RI fatalities, Small numbers problem for 19861989 individual states 13% increase in UI fatalities, Weak information on total 19861989 crashes and nonfatal injuries Monthly changes mirror annual changes
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1988 exposure (i.e., trend term) Estimated median effect of law on Large differences across indi non-RI fatalities -- 5% increase vidual states Only considered fatalities (continued on next page)
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Thirty-eight 65- Before/after odds Odds ratio assumes indepen a states mph ratio 15% increase in RI fatalities rela- dent series 1989 Eight 55-mpha states 19821986 versus tive to other rural roads (odds Similar results for uniform versus Annual data: 1987 ratio = 1.15) dual speed limit laws, safety 55-mpha states 19821987 belt/no safety belt law, day No effect on RI odds ratio (odds time/nighttime crashes, and ratio = .94)
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b, 1990 ratio Odds ratio assumes indepen a states Eight 55-mph 29% increase (19% after adjust 1991b 19821986 versus dent series Annual data: ments for VMT and passenger 1988 Adjusted for VMT increases 19821989 vehicle occupancy rates) in RI 19821986 versus and changes in vehicle occu fatalities relative to other rural Effect of Speed Limits on Speed Distributions and Highway Safety 1989 pancy roads (odds ratio = 1.29)
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Godwin Thirty-eight 65- Before/after analysis Decreased fatalities systemwide Comment on Lave's (1992) 1992b mpha states implies unreasonably high VMT finding Eight 55-mpha states shift from non-RI to RI roads Other than VMT, no control Annual data: 1986, for confounding factors 1988 How comparable is the com parison group?
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1990 Note: AADT = average annual daily traffic; RI = rural Interstate; UI = urban Interstate; VMT = vehicle miles traveled.
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II-7) , the basic assumption is that the rural Interstate series and the companion series move well enough together historically that a deviation in the historical pattern can be interpreted as the result of a higher speed limit in 1987.
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(1989; 1990; 1991) used a before-and-after analysis based on odds ratios to assess the effect of higher RI speed limits on highway safety.
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. Similar results were found for comparisons of 55-mph states with and without dual speed limits, states with and without safety belt laws, and daytime versus nighttime crashes.
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Garber and Graham's finding of positive and negative effects of the speed limit on statewide fatalities is consistent with the notion that existing studies have not sufficiently identified or controlled for the influence of other factors on state fatalities. If a model adequately accounted for these differences, the speed limit law could be expected to have the same effect on highway safety in different states.
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. Overall, the authors concluded that the 65-mph speed limit had a statistically significant effect on fatalities, but after a year's "learning period" the effect decayed over time.
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speed limit, most but not all of which are for the more populous states. Similar to the national studies, the state-specific analyses use a variety of methodological approaches, but the state-specific studies are typically more general in that many examine the effects of the speed limit on fatal as well as nonfatal measures of highway safety.
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From page 328... ...
1978Dec. vention analysis founding effects but some 1989 1988 40% increase in serious injuries on are significant with perverse 65-mpha roads Monthly signs 38% increase in fatalities on 55- Models variable levels only mpha roads Large confidence intervals Fatality equation: R2 = .03 Limited postenactment sample Streff and Michigan ARIMAX inter- 28% increase in fatalities Extends Wagenaar et al.
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1983July 1988 tion analysis car crash rate Interstate crashes Monthly Decrease in fatal-injury car-truck How sensitive are results to crash rate intervention month? McCarthy Indiana Regression analysis Increase in total, fatal/injury, and Fixed effects model 1993 Countywide data PDO alcohol-related crashes Controlled for exposure, age 19811989 Redistribution of alcohol-related distribution, population, crashes from higher-speed to economy, alcohol availabil lower-speed environments ity, and enforcement Similar effects for most cate gories of alcohol-related crashes (e.g., daytime, single-vehicle, non-truck involved)
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1994 19851987 versus sis Decrease in systemwide fatalities No control for confounding 19891992 ANOVA factors Effect on RI fatalities stabi lized in 19901991 Dual speed limit had no effect on car-truck crashes Khorashadi California Before/after analy- Increase in fatal crashes on 65- Fixed versus random effects RI 65-mpha roads, mpha roads, RI and RNI 1994 sis ANOVA 1,155 mia ANOVA Increase in total, fatal, injury, and Examined various crash RI 55-mpha roads, fatal and injury crashes on 65- causes but no control for 343 mia mpha roads relative to 55-mpha VMT or other confounding RNI 65-mpha roads effects roads, 132 mia 19821986 versus 19881992
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over fatal crashes on Interstate the same period. McCarthy roads, U.S.
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Table C-5 (continued) Study Database for Study Methodology Major Findings Comments Rock 1995 Illinois ARIMA interven- 40% increase in RI fatalities, 65- Modeled levels mpha roads May 1982April tion analysis Abrupt permanent interven 1991 25% increase in RI fatalities, 55- tion mpha roads Monthly Considered only rural high ways Note: PDO = property damage only; RI = rural Interstate; RNI = rural non-Interstate; VMT = vehicle miles traveled.
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speed limit, commented that the new law could have actually saved lives. Using data from an NHTSA (1989b)
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. Lave attributed the reduction to the 65-mph speed limit since fatality rates in the 55-mph states remained constant.
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Two of the more important concerns raised by Griffith, relative to the time series regression results, and Lave's responses are as follows: · Concern: Statewide fatality rates may be too broad to capture systemwide effects of the speed limit. Response: The error associated with too narrow a definition of systemwide effects will miss some of the law's effects, whereas too general a definition will reduce the model's explanatory power by adding noise to the model.
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H1: The 65-mph speed limit increased or decreased systemwide highway safety. If the direct and spillover effects of the law are emphasized to the exclusion of the traffic diversion effects, highway safety is expected to
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· What role do other confounding factors not included in Lave and Elias's models have on the systemwide highway safetyenhancing effects of the 65-mph (105-km/h) speed limit law?
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However, part also reflects differences across states that are not adequately accounted for in the empirical models. · Similar to the law's mixed effects on the speed distributions of nonlimited-access roads, the higher RI speed limits have produced mixed highway safety effects on nonrural Interstate roads.
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1987 the same before and after the law change General decrease in speeds and Prior speed limits were 31 mpha Schleicher- Germany Before/after Jester Implemented 19- analysis crash severity Speed limit decreases combined mpha speed zones 1990 with public information, traffic 19831986 control, speed control, and street design changes (continued on next page)
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Netherlands, 15 Before/after 20% speed reduction, generally No information on prior speed 1992 municipal analysis resulting in an 85th per- limits centile speed of 19 mpha areas Quasi-experi- Speed limit aimed to "integrate" Implemented 19- mental Traffic volume fell 5% to 30% road user categories, where the mpha speed zones motorist identifies 19 mpha as 5% trend-adjusted decrease in 1980s all crashes the appropriate speed 25% trend-adjusted decrease in Combined with engineering mea injury crashes sures to slow traffic (e.g., humps, axis realignments, traffic islands) Where did the decreased traffic go?
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Engel and Denmark, residential Quasi-experi- 18.4% decrease in control No information on prior speed Thomsen areas mental group adjusted crashes limits Introduced 19-mpha Before/after 1992 21.1% decrease in control 3 years of before data, 3 years of speed zones analysis group adjusted injuries after data 139 mi,a experimental group; 44 experimental Regression analy- 72% decrease in casualties per 11,766 mi,a control group areas, 53 control sis road user, experimental areas areas No change in crash risk per Status of streets changed from 1980s user in experimental areas "traffic streets" to "living areas" 96% increase in casualties per Speed-reducing measures also road user, just outside experi- implemented 0.4-in.a increase in height of hump mental areas decreased speed by 0.6 mpha Road narrowing decreased speed by 2.9 mpha No discussion of effect on casual ties per road user in outer areas (continued on next page)
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342 Table C-6 (continued) Study Database for Study Methodology Major Findings Comments MANAGING SPEED 25-mpha limit led to permaCairney City of Unley, Before/after No information on prior speed nent 3-mpha reduction in and Australia analysis limits 25-mpha speed zone 2-mia by 660-fta study area in Fackrell speed 1993 19911993 Initial temporary fall in traffic Unley volume Size of speed reduction at experi Effect of increased enforce- mental sites varied ment ambiguous Examines effects of speed limit changes with and without speed camera enforcement Authors question whether changes in economic factors may have affected results Newstead Victoria, Australia Before/after No systemwide effect Speed limits increased on 1,196 31-,a 43-,a and 50- mia of roads, decreased on 342 and analysis 6.9% increase in injury crashes mpha speed limit mia of roads Mullan Quasi-experimen- for metropolitan Melbourne, 1996 zones tal marginally significant For Melbourne, 47% decrease in 19921993, 32.9% reduction in injury injury crashes when limit increased from 37 mpha to 50 mpha and a 19941995 crashes in the rest of Victoria, marginally signifi- 10.5% increase when limit increased from 47 mpha to 50 mpha cant No control for other confounding factors
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aStudy used Standard International units, which were converted to U.S. equivalents in the table.
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From page 344... ...
, neither effect statistically changes in public informa significant tion and enforcement or other confounding factors Controlled speed limit was 62 mpha Sliogeris Victoria, Australia Before/after Statistically significant 24% increase in 1992 Imposition and analysis injury crashes per mile after introduc- No control for other factors tion of 68-mpha speed limit removal of a Regression Control group is all other 62 68-mpha speed mpha signed roads in analysis Statistically significant 19% decrease in limit injury crashes per mile after removal Victoria of 68-mpha speed limit 19851991 Similar results for rural and urban roads
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From page 345... ...
Borsje Netherlands Before/after Differentiated speeds on motorways 75 mpha on 80% of motor 1995 Introduction of analysis decreased average speed and had a ways, 62 mpha on 20% of general 75-mpha nonincreasing effect on speed disper- motorways sion for 62-mpha and 75-mpha roads speed limit Statistical significance of 19881992 Positive effect on crash incidence results not reported Accompanying policies included greater enforce ment, media campaigns, infrastructure changes Johansson Sweden Poisson time No statistically significant effect on fatal Methodology accounts for 56-mpha speed 1996 series analy- or serious injury crashes overdispersion and serial limit sis Statistically significant decrease in minor correlation Monthly, injury and vehicle damage crashes Controls for exposure 19821991 (through economic vari ables) , seasonal effects, safety belt law aStudy used Standard International units, which were converted to U.S.
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From page 346... ...
However, there were differential effects, depending on the specific combination of actions taken, which suggests that effective speed limit changes involve the implementation of reinforcing policies. Faure and de Neuville (1992)
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Higher-Speed Roads The relatively few recent international studies on higher speed limits, reported in Table C-7, indicate a positive correlation between relaxed speed limits and average speeds and crashes. Nilsson (1990)
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in speed limits were associated with decreases (increases) in average speed and speed dispersion.
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CONCLUSIONS AND AREAS FOR FUTURE RESEARCH The purpose of this review is to provide an overview of recent empirical research on the effect of changes in posted speed limits on speed distributions and highway safety. To provide a contextual basis for the
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From page 350... ...
Conclusions With regard to specific implications for speed limit policy, existing studies provide support for the following conclusions: · Speed limits must be perceived by the traveling public as "reasonable," that is, as consistent with the enforced traffic conditions experienced by the typical driver. To the extent that this is not true, more drivers will be noncompliant, which could compromise highway safety.
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From page 351... ...
Areas for Future Research There are a number of areas in which additional research could significantly contribute to an understanding of the effect of speed limits on speed distributions and highway safety. These areas are discussed in the remainder of this section.
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From page 352... ...
Also, since travel occurs on a network of interconnected roads, are existing aggregate measures of highway safety reasonable speed distribution proxies for different functional road types, or does use of these measures cause important biases? Methodology As noted earlier, existing research on speed limits generally uses univariate classification procedures, regression analysis, or ARIMA time series models.
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From page 353... ...
· How important is enforcement in determining speed distribution properties and highway safety when speed limits change? · Although there is some information on the relationship between average speed, speed dispersion, and highway safety, much additional work is needed to improve the understanding of these relationships.
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From page 354... ...
It is not known whether these effects exist and, if so, how important they are in determining the highway safety effects of speed limit changes. · Sufficient evidence exists to question whether the net effect on highway safety of speed limit laws is to deteriorate highway safety.
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From page 355... ...
1989. The Mortality Consequences of Raising the Speed Limit to 65 mph on Rural Interstates.
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From page 356... ...
Speed Limit on Highway Safety in the U.S.A. With comments by C
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From page 357... ...
1994a. An Empirical Analysis of the Direct and Indirect Effects of Relaxed Interstate Speed Limits on Highway Safety.
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From page 358... ...
1989. The 65 mph Speed Limit in Michigan: Effects on Injury and Death.
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