Managing Speed Review of Current Practices for Setting and Enforcing Speed Limits -- Special Report 254 (1998) / Chapter Skim
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Chapter 2: Effects of Speed
Chapter 2: Effects of Speed
Pages 36-76
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From page 36... ...
In this chapter, what is known about the relationships among speed, crash incidence, and crash severity is reviewed. Individual driver decisions about appropriate travel speeds, however, are guided by more than safety considerations.
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From page 37... ...
DETERMINATION OF APPROPRIATE DRIVING SPEEDS -- MAKING TRADE-OFFS How do people decide how fast to drive? Many factors come into play including the characteristics of the road; the amount of traffic on the road; weather conditions and time of day; the speed limit and its enforcement; the length and purpose of the trip; the vehicle's operating characteristics, such as handling and stopping as well as fuel consumption and emissions; and driver-related factors, such as the propensity to take risks and the pleasure associated with driving fast.
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From page 38... ...
There is reason to believe, therefore, that where speed choice is not constrained by speed limits and their enforcement, the driver does trade off travel time and safety. Even when visibility and weather conditions are good, drivers may still make trade-offs.
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From page 39... ...
The definition of speeding is broad; for the purposes of coding crash-related information, speeding is defined as "exceeding the posted speed limit or driving too fast for conditions" (NHTSA 1997a, 1)
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From page 40... ...
and the time when action must be taken to avoid a crash.5 A crash is likely to occur when the information processing demands exceed the attentional or information processing capabilities of the driver (Shinar 1978) .6 Unexpected events dramatically increase information pro 4 Note that the 20 percent figure refers to the share of drivers involved in speedingrelated fatal crashes as a percentage of drivers involved in all fatal crashes, whereas the 30 percent figure cited earlier refers to the share of speeding-related fatal crashes as a percentage of all fatal crashes.
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From page 41... ...
Drivers with speeds much faster or much slower than the median traffic speed are likely to encounter more conflicts (Hauer 1971) .7 This relationship leads to the conclusion that "speed deviation kills" and the prediction that on roads with equivalent average traffic speeds, crash rates will be higher on roads with wider ranges of speed.
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From page 42... ...
In the latter case, when referring to the distribution of speeds in a traffic stream, three measures of speed are typically considered: the average speed, the 85th percentile of the speed distribution, and the dispersion in travel speeds. Speed dispersion, in turn, can be quantified by the variance, standard deviation, 10-mph pace, or range (high minus low)
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From page 43... ...
on 28 out of 35 sections Cirillo Compared speeds of crash- Same finding as Solomon, but crash involvement rates were Rural and urban Interstate (1968) involved vehicles with speeds lower for all travel speeds suggesting importance of roadway highways, U.S.; no of non-crash-involved vehi- geometry to crash probability (i.e., higher design standards speed limits given cles; limited to daytime on Interstate highways than on rural two-lane roads)
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From page 44... ...
After controlling Interstates, arterials, and dispersion (measured as 85th for speed dispersion, average traffic speed not significantly collectors; data from 50 percentile speed minus 50th related to fatality rates for any road type states percentile speed) , and two nonspeed measures -- traffic citations per driver and access to medical care -- on fatality rates Garber Higher-speed roads [i.e., Analyzed relationship between Crash rates increased with increasing speed variance on all road and with average traffic crash rates and average traf- classes.
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From page 45... ...
undivided roads with ticipated in a road safety sur- Self-reported crash involvements were lowest for those trav speed limits of 62 mph vey eling at speeds below average traffic speeds and highest at (100 km/h) , Australia speeds above the average with no advantage at the average Baruya Urban roads with average Analyzed relationship between Both speed level and speed dispersion affected crashes.
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From page 46... ...
His well-known U-shaped curve (Figure 2-1) showed that crash involvement rates are lowest at speeds slightly above average traffic speeds.
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From page 47... ...
and 4 p.m. on the assumption that the effect of deviation from average traffic speeds on crash involvement could best be determined by examining crashes involving vehicles traveling in the same direction (Cirillo 1968, 71)
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From page 48... ...
together with Indiana University (RTI 1970) addressed several of these issues by using speed data based, in part, on traffic speeds recorded at the time of the crash.19 They examined crashes on highways and county roads with speed limits of 40 mph (64 km/h)
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From page 49... ...
Deviation from average traffic speeds, however, is not the only factor linking speed with crash involvement. It does not explain, for example, the significant fraction of speeding-related driver involvements in fatal crashes involving only one vehicle -- nearly 50 percent in 1996.20 In fact, when Solomon's data are disaggregated by crash type, the U-shaped relationship is only fully replicated for one crash type -- nighttime headon collisions (Cowley 1980 in Cowley 1987, 9)
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From page 50... ...
. Disaggregation of Solomon data for nonlimited-access rural highways.
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From page 51... ...
Crash involvement rates rose linearly as a function of speed; crash involvements were lowest at speeds below average traffic speeds and highest at speeds above the average with no advantage at the average (Fildes et al.
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From page 52... ...
. The data showed a steady and statistically significant increase in the probability of involvement of the case vehicles in a casualty crash with increasing speed above, but not below, the 37-mph speed limit, which roughly approximated the average traffic speed.
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From page 53... ...
, confirmed the importance of speed dispersion to fatality rates but also found that average traffic speed is an important determinant.29 None of the studies discussed in this paragraph examined differences in roadway design features or traffic levels within road class, which could affect traffic speeds and crash rates. 27 Lave defined speed dispersion as the difference between the 85th percentile speed and the average traffic speed.
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From page 54... ...
Garber and Gadiraju examined the relationship between crash rates, speed dispersion,30 average traffic speed, and other measures that influence speed -- design speed and posted speed limits -- on several different classes of roads in Virginia.31 They found that crash rates declined with an increase in average traffic speeds when data for all road classes were combined (Garber and Gadiraju 1988, 26)
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From page 55... ...
. Cirillo found, as Solomon had before, an association between crash involvement rates and deviation from average traffic speeds even on Interstate highways.
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From page 56... ...
(1991) showed a gradual increase in crash probability for vehicles traveling above, but not below, average traffic speeds on two-lane rural roads (Figure 2-1)
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From page 57... ...
Offsetting these effects to some extent is the fact that congestion tends to reduce driving speeds, thus lessening the severity of the crashes that do occur. Some studies of the relationship between speed and crash probability on urban arterials found a link between speed deviation and crash involvement for vehicles that travel at speeds well above average traffic speeds.
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From page 58... ...
. Finally, a recent study of traffic speeds and personal injury crashes on urban roads in Great Britain, which classified roads by speed-related variables, found that measures of speed dispersion and speed levels have counterbalancing effects (Baruya and Finch 1994, 228)
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From page 59... ...
integrated three large data files to obtain more reliable estimates of the role of speed in crash causation.38 Although they were based on different data sets and methodologies, the three sources yielded similar estimates, with "excessive speed" reported as being involved in approximately 12 percent of all crashes and more than 30 percent of fatal crashes (Bowie and Walz 1994, 31)
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From page 60... ...
. The second most frequently cited crash cause -- responsible for 11 percent of the fatal crashes -- was labeled "rocket-ship." It involved single-vehicle, frontal-impact crashes with the "vehicle leaving the road at a very high speed." Because the analysis was confined to fatal crashes of belted occupants -- and unbelted occupants are more highly represented in fatal crashes -- the percentage of speeding-related fatal crashes in the population at large is certain to be higher.
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From page 61... ...
Summary Although the evidence is not conclusive, speed appears to contribute to crash occurrence. Theory, empirical data drawn from correlational studies, and causal analyses of crashes provide evidence that both speed and speed dispersion are associated with crash involvement.
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From page 62... ...
Limited data are available to analyze speed-safety relationships by road class. Deviation from average traffic speeds appears to play a role in crash involvement on Interstate highways, particularly near interchanges on urban Interstates, and to a greater extent on rural nonlimited-access highways where high vehicular speeds and poorer road design combine to increase crash probability.
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From page 63... ...
Using three measures of crash severity -- deaths, injuries, and property damage per involvement -- the study showed that the higher the speed, the greater the 41 The equation that describes the release of kinetic energy as it relates to vehicle mass and speed is as follows: kinetic energy = 0.5 mass (velocity)
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From page 64... ...
However, the association between higher speeds and higher crash severity levels that Solomon found has been borne out in subsequent studies. Several other researchers have confirmed the consistent relationship between speed and injury severity in crashes.
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From page 65... ...
. Combining several different crash files, the authors also compared injury severity levels with the distribution of injuries in speeding-related crashes.
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From page 66... ...
RELATIONSHIP OF SPEED TO TRAVEL TIME In addition to safety, travel time is a major factor affected by speed that influences drivers' choice of an appropriate driving speed. The importance and cost of travel time as a function of speed were amply illustrated by the recent experience of the 55-mph (89-km/h)
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From page 67... ...
. The travel time costs to motorists on other road classes were estimated to have much smaller effects, in part a reflection of the role of congestion and roadway geometry in limiting travel speeds on these nonlimited-access highways.45 Given these results, it was not surprising that the relaxation of the NMSL first occurred on rural Interstate highways.
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From page 68... ...
For many work trips, congestion is likely to have more effect on driving speeds and travel time than are reductions in speed limits. Most personal travel (68 percent in 1990)
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From page 69... ...
, based on a small sample of 1988 to 1995 model year automobiles and light-duty trucks, shows a clear relationship between fuel economy and driving speed. Under steady-state, cruise-type driving conditions, fuel economy peaks at about 55 mph (89 km/h)
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From page 70... ...
Thus, they are not likely to affect motorists' choice of appropriate driving speeds. RELATION OF SPEED TO EMISSIONS Speed is clearly linked with vehicle emissions that contribute to pollution of the atmosphere, particularly to the degradation of metropolitan air quality.
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From page 71... ...
drivers do not directly pay for the costs that this pollution imposes on society.49 Thus they are not apt to consider environmental costs in their choice of an appropriate driving speed. SUMMARY In this chapter, the role of speed has been considered as it relates to the major factors motorists take into account in determining appropriate driving speeds.
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From page 72... ...
Vehicle-pedestrian crashes are an exception, because pedestrian injuries tend to be severe even at low impact speeds. Both speed and speed dispersion appear to play a role in crash likelihood on urban arterials; speed deviation above average traffic speeds and higher speeds in general are closely linked with crash probability on these roads.
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From page 73... ...
First, the unambiguous relationship between speed and crash severity alone is sufficient justification for controlling driving speeds. Second, if they are enforced, speed limits -- the most common method of managing speed -- can help restrict travel speeds, particularly at the very high speeds where the injury consequences of crashes are the greatest.
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From page 74... ...
1994. Investigation of Traffic Speeds and Accidents on Urban Roads.
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From page 75... ...
1997. Effects of Raising and Lowering Speed Limits on Selected Roadway Sections.
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From page 76... ...
1971. Accidents, Speed Deviation and Speed Limits.
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