Managing Speed Review of Current Practices for Setting and Enforcing Speed Limits -- Special Report 254 (1998) / Chapter Skim
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Appendix B: Speed and Crashes: A Controlversial Topic and an Elusive Relationship
Appendix B: Speed and Crashes: A Controlversial Topic and an Elusive Relationship
Pages 221-276
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From page 221... ...
In fact the feeling was one of "sameness." What I think had changed in the interim was the perceived enforced speed limit [raised from 55 to 65 221
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That axiom is encapsulated in the slogan "speed kills." Speed is also listed as one of the manifestations of "aggressive driving" by the National Highway Traffic Safety Administration (NHTSA) (Martinez 1997)
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First there is a need to distinguish between speed limits (prescribed speed) and travel speeds (drivers' speed)
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Three such statistics are most often used in the context of safety: average travel speed, 85th percentile of the speed distribution, and some measure of the dispersion in travel speeds. Speed dispersion, in turn, can be quantified by the speed variance (the squared deviations from the mean of the speed distribution)
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From page 225... ...
found that drivers at precrash speeds significantly above or below the average traffic speed have a greater likelihood of overinvolvement in crashes than do those driving just slightly above the average (Solomon 1964)
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However, since these techniques are also used as surrogate measures of travel speed, a brief review of this relationship is appropriate. Speed Limits The impact of speed limits on crash risk is addressed in Appendix C
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suggested that the difference between the design speed and the posted speed limit accounts for differences in driving speeds; widening speed dispersion, in turn, was linked to increases in crash rates. On the Virginia highways they studied, minimum speed dispersion was obtained when the design speed was 5 to 10 mph (8 to 16 km/h)
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From page 228... ...
. Parker's analysis is relevant here because it measures the effects of changes in speed limits on driving speeds and the effects of these changes on crash involvement.
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From page 229... ...
, then -- either because of their high speed or because of their contribution to widening the range of traffic speeds -- speeding drivers are at a greater crash risk. Their increased risk is consistent with both the "speed" model and the "variance" model because the more the driver exceeds the average traffic speed, the greater the range in traffic speeds and the further the driver's position from the minimum point of the U-shaped crash involvement curve.
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demonstrated that massive enforcement, with a daily average of police presence of 9 h, yielded speed reductions that lasted up to 8 weeks. This was done in a semirural area with a road section having speed limits of 37 and 50 mph (60 and 80 km/h)
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. SPEED AND CRASH INVOLVEMENT From a very simplistic point of view it appears that as speed increases, the time to react to emerging dangers is shortened, and the likelihood of successfully coping with the imminent crash situation decreases.
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The limit is on the rate of information processing. From a theoretical perspective, if a driver is assumed to be introduced into a fixed roadway/traffic situation, then the faster that driver drives, the greater the required rate of information processing, and the greater the demands of maneuverability of the car in an imminent crash situation.
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From page 233... ...
If attention level remains constant despite increases in speed, then the crash potential due to a lapse in attention for a given duration increases as speed increases because in that duration the distance traveled increases and the safety margin decreases. If, on the other hand, the driver increases the amount of attention with increasing speed (as some drivers claim -- the extreme claim being that driving slowly is boring and induces drowsiness)
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From page 234... ...
. This logic leads to the conclusion that it is not speed that kills but the deviation from the median or average traffic speed that kills; the more a driver contributes to this deviation (by driving faster or slower than the median or average)
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This means that increasing speed per se is not a dangerous behavior but that an inappropriate excessive speed -- stemming from misperception of the situational demands and lack of appreciation of the car and the driver's own handling capability -- may be dangerous. This approach can lead to different predictions concerning the relationship between speed and crashes, but since most drivers' perceptions of the road and traffic ahead are fairly accurate, it would predict that under most circumstances the voluntary increase in speed of most drivers would not necessarily increase crash risk.
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From page 236... ...
study on the effects of raising and lowering speed limits on selected nonlimited-access roadway sections, the percentage exceeding the posted speed limit is typically greater than 20.8 percent. For this reason, to evaluate accurately the contribution of speed to
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. The second analysis was detailed and focused on Virginia crashes; it failed to find any significant relationship between average speed and crash rates (Garber and Gadiraju 1988)
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Because of the care that Solomon took in examining all three aspects of speed -- average speed of the traffic stream, speed dispersion, and reported speed of crash-involved vehicles -- and because Solomon's study was the first, and to date arguably the most detailed and comprehensive study of this nature, its essential design features are described before its findings and conclusions are reported. The study analyzed the crash experience of 10,000 driver-vehicles that had been involved in crashes between 1954 and 1958 on 600 mi (1000 km)
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Solomon hypothesized that the speed with the lowest crash rates should correspond roughly to the average traffic speed. Seven years after Solomon's study was published, Hauer (1971)
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1 mph = 1.609 km/h. average traffic speed is lower than the median, and the 5- to 10-mph minimum point above the average corresponds fairly well to Hauer's (1971)
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In a related analysis, Solomon studied crash involvement of pairs of passenger vehicles involved in rear-end collisions. He found that crash-involved pairs were much more likely to travel at larger speed
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Two additional issues appear to have been overlooked in the previous critiques of Solomon's conclusions and others' interpretations of his results and should be added to the limitations. First, in arriving at his conclusion, Solomon makes the subtle substitution of a cause-and-effect relationship for the observed association between the speed deviation from the average traffic speed and crash involvement.
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From page 243... ...
Unlike many other measures of driver performance in which the variance increases with increases in the mean, increases in speed limits typically result in smaller increases in the average speed and no consistent increases in measures of speed dispersion (Brown et al. 1990; Freedman and Williams 1992 for freeflowing vehicles on expressways; McCarthy 1991)
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. In both of the studies that obtained this relationship, speed dispersion was defined in terms of the traffic speed distribution (and not in terms of the deviation of crash-involved vehicles from the average traffic speed)
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, with recorded average speeds ranging from 42 to 59.5 mph (68 to 95.8 km/h) , crash rates actually declined with increasing average speed in a logarithmic fashion.
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Predictably, crash rates increased with speed for single-vehicle run-off-the-road crashes and decreased with speed for rear-end crashes. However, angle collisions, "single vehicle struck object" crashes, and daytime-only head-on collisions decreased with increasing speed, suggesting that there is something to the argument that slow or slowing vehicles are overinvolved in crashes without necessarily shedding light on why this is so.
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The researchers used the same measures as Solomon for crash involvement and for speed differences (deviation of crash vehicle from the presumed average traffic speed)
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and on two urban segments [four-lane undivided arterial roads with posted speed limits of 37 mph (60 km/h)
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This may create imminent crash situations because of lapses in attention of following drivers, slowed responses of the following drivers, or misperception of the reduced gap by the following drivers. Lapses in attention (variously labeled as inattention, distraction, or improper lookout)
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With 34 to 40 mph (55 to 64 km/h) used as the reference speed, increased crash involvement was obtained only for drivers exceeding the speed limit and not for those traveling at speeds less than the speed limit.
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cars with precrash speeds that are either significantly above or below the modal or average travel speed are likely to be overinvolved in crashes; and (d) at least part of the overinvolvement of slow vehicles is due to forced slowing down such as for intersections, avoidance of obstacles, and so forth.
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From page 252... ...
This approach to coding "speeding" as a causal factor is different from the FARS coding, where all causal or related factors are based on police crash reports and thus speeding as a crash cause is likely to include a mix of speeding relative to the posted speed limit and relative to prevailing conditions. More recently, the role of speed in the causation of fatal crashes was assessed by Viano and Ridella (1996)
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From page 253... ...
(If the percentage of drivers speeding in the traffic stream -- in the clinical sense, not relative to the speed limit -- is greater than the percentage of speeders in crashes, it could be argued that speeding may be a mitigating factor in crash involvement.) Importance of Road Type People drive differently in different environments -- on different road types, on roads with different design speeds, and [to a lesser extent
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From page 254... ...
analyzed the effects of road type and design speed on speed and crashes and found a strong relationship between road type and design speed and between these two variables and crash rates. Their data consisted of the average traffic speed, speed variance, and number of crashes at 36 road sections consisting of different road types with different design speeds.
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, the effect of the speed of crash-involved vehicles on crash probability could not be directly tested. Models of linear regression of the average traffic speeds on different road sections against their crash rates yielded no significant correlations.
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, the correlations between both variables and crash rates were statistically nonsignificant. Thus, no significant effects of average speed or speed dispersion were found for urban Interstates in either year.
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Since 12 out of the 14 arterial sections in their study were rural, it is safe to attribute the finding to rural arterial highways. Lave obtained a weak but statistically significant relationship between crash rates and a measure of traffic speed dispersion (85th percentile minus average traffic speed)
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From page 258... ...
In summary, there appears to be a limited amount of data -- and with many methodological shortcomings mentioned in the preceding section -- that suggest that on Interstate highways, increases in the range of traffic speeds are associated with increased crash rates; the more a vehicle deviates from the average traffic speed (or from slightly above the average traffic speed) , the more likely it is to be involved in a crash.
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Crash severity can be defined in at least two ways: (a) the physical severity of the impact speed or Delta-V (the velocity change in the crash)
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. By fitting curves to crash data with known and estimated Figure B-4 Persons injured per 100 involvements versus travel speed for daytime crashes (Solomon 1964)
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A relationship where the dependent variable is a function of the independent variable taken to some power is known as a power function. In the present analyses of the relationship between severity (the dependent variable)
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It is next to impossible to examine the original data that both used, but it appears that O'Day and Flora's estimates are more appropriate, because 1995 NASS data (with national safety belt use rates exceeding 60 percent) indicate a fatality rate of 33 percent for Delta-Vs above 45 mph (72 km/h)
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An appreciation of the societal costs of crashes of various severity levels is possible when crash rates are disaggregated and their inci
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The daytime and nighttime curves in this figure are similar to the crash rates in Figure B-1 but (a) have their minimum at a lower speed level [approximately 55 mph (89 km/h)
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than was observed for all crashes in Figure B-1. An even sharper trend (above the average speed)
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rates by travel speed, day and night (Solomon 1964)
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CONCLUDING REMARKS There is sufficient evidence to indicate that a driver's absolute speed is a correlate of crash involvement. The indications for the positive relationship between speed and crashes are derived from empirical data of single-vehicle crashes, causal crash analysis, and theoretical frameworks related to the effects of speed on information overload and reduced vehicle-handling capacity.
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From page 268... ...
In a situation in which speed selection is totally at the driver's discretion, the range of speeds in the traffic stream is a function of the risk levels that different drivers are willing to tolerate, different perceptions of a "safe speed" that drivers have for a given risk level, and the handling capabilities of different cars and drivers. All of the studies reporting narrowing of speed disparities with increasing speed were conducted in the presence of speed limits, and, consequently, a threshold level of speed may have been responsible for the reduction in speed dispersion (i.e., higher speeds were due to higher speed limits on roads with higher design speeds)
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From page 269... ...
If speed limits are set at low levels and they are enforced, then speed dispersion would probably not decrease with increasing speed but rather would increase with it, in a manner similar to most measures of human psychomotor behaviors (where variance is positively correlated with the mean)
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From page 270... ...
2. There are no convincing data to demonstrate that, across all roads, crash involvement rates rise with the average speed of traffic (i.e., that roads with higher average traffic speeds have higher crash rates than roads with lower average traffic speeds)
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From page 271... ...
7. The data demonstrating the relevance of speed dispersion in the traffic stream and speed deviations of crash-involved vehicles are based on correlational effects and therefore cannot be used to indicate that if slow-moving drivers were to increase their speed, their crash probability would be reduced.
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From page 272... ...
1993. The Speed Review: Road Environment, Speed Limits, Enforcement, and Crashes.
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1992. Speeds Associated with 55-mph and 65-mph Speed Limits in Northeastern States.
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From page 274... ...
1993. Evaluation of Lower Speed Limits on Urban Highways.
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From page 275... ...
1997. Traffic Speeds Following Repeal of the National Maximum Speed Limit.
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From page 276... ...
1971. Accidents, Speed Deviation and Speed Limits.
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