Identification of Vehicular Impact Conditions Associated with Serious Ran-off-Road Crashes (2010) / Chapter Skim
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Pages 26-54
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From page 26... ...
The modest changes observed in vehicle weight and roadway speed limits could be explained by changes in the vehicle fleet and elimination of the national speed limit law. Unfortunately, the magnitude of the change in vehicle class and the rollover rates between the 17-22 and TTI data could not be adequately explained.
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From page 27... ...
Figure 3. 17-22 and TTI heading angle distributions.
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From page 28... ...
As would be expected for crashes collected from these highway types, the data set includes a wide distribution of speed limits ranging from 45 to 75 mph, as shown in Table 17. Table 18 presents this distribution of speed limit by highway class.
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From page 29... ...
Almost 58% of vehicles included in the data set were classified as "car." Further, another 28% of vehicles fell into the compact light truck class including compact pickups, compact utility vehicles, and minivans. Only 13% of vehicles included in the database were full-size pickups, utility vehicles, or vans.
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From page 30... ...
These fatality and A+K rates were 25 and 19 times higher, respectively, than the values for controlled-access freeways in Kansas. This degree of bias is associated with the original case-selection criteria used to identify the NASS CDS cases and therefore cannot be avoided.
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From page 31... ...
of Cases Fatal A-injury B-Injury C-Injury PDO Subcompact 145 13.1 53.8 11.0 12.4 9.7 Compact Car 167 16.2 52.1 10.8 10.2 10.8 Intermediate 117 13.7 63.2 9.4 6.0 7.7 Full-Size Sedan 55 14.5 54.5 12.7 10.9 7.3 Car Large Size 22 4.5 68.2 13.6 0.0 13.6 Compact Pickup 99 19.2 57.6 7.1 8.1 8.1 Large Pickup 87 10.3 64.4 5.7 10.3 9.2 Pickup Truck Other Pickup Type 3 33.3 33.3 0.0 0.0 33.3 Compact Utility 120 14.2 59.2 14.2 5.0 7.5 Large Utility 8 0.0 75.0 12.5 0.0 12.5 Utility Vehicle Stationwagon Utility 15 13.3 60.0 6.7 13.3 6.7 Minivan 27 22.2 63.0 7.4 3.7 3.7 Large Van 2 50.0 50.0 0.0 0.0 0.0 Van Full-Size Van 10 30.0 50.0 10.0 10.0 0.0 Table 28. Crash severity by vehicle class.
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From page 32... ...
Table 35 shows the relationship between impact velocity and crash severity for W-beam guardrails. Again, there appears to be a strong correlation between impact speed and probability of fatal and serious injury.
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From page 33... ...
Crash severity vs. impact speed for W-beam guardrail.
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From page 34... ...
In the late 1970s, very few passenger cars had antilock brakes and by the late 1990s, the majority of the vehicle fleet was so equipped. In theory, antilock brakes are intended to allow drivers to continue to steer through emergency braking procedures.
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From page 35... ...
found that the speed data fit a normal distribution. As a first step to modeling departure conditions, normal and gamma distributions were fit to departure speed and angle data for the total database and for each speed limit range as shown in Tables 47 and 48.
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From page 36... ...
Chi Squared – Gamma Speed Limit (mph)
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From page 37... ...
Tables 47 and 49 provide parameters for fitting normal and gamma distributions to departure speed and square root of departure angle, respectively. The next step in modeling departure conditions involved exploring the dependence of speed and angle.
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Observed departure conditions. Departure Angles (deg.)
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4.4.1 Impact Speed and Angle Distributions Table 61 compares departure conditions to impact conditions for the first harmful event. Notice the significant change in velocity from roadway departure to the first impact.
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Departure condition distribution for 70 mph speed limits. Velocity (mph)
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Departure Angle Range 0o - 5o 5o - 10o 10o - 15o 15o - 20o 20o - 25o 25o - 30o >30o <20 0.00284 0.00634 0.00566 0.00411 0.00279 0.00184 0.00357 20 - 30 0.00939 0.02093 0.01870 0.01359 0.00922 0.00609 0.01181 30 - 40 0.02165 0.04827 0.04312 0.03134 0.02125 0.01405 0.02723 40 - 50 0.02983 0.06651 0.05942 0.04319 0.02929 0.01935 0.03752 50 - 60 0.02457 0.05479 0.04895 0.03557 0.02413 0.01594 0.03091 60 - 70 0.01210 0.02697 0.02410 0.01751 0.01188 0.00785 0.01522 >70 0.00425 0.00947 0.00846 0.00615 0.00417 0.00276 0.00534 Table 58. Departure condition distribution for 50 mph speed limits.
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From page 42... ...
The T-test findings indicated that segregating impact speed data by highway class may be more appropriate than segregation by speed limit range. Table 64 shows descriptive statistics for impact angle segregated by both highway class and speed limit.
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From page 43... ...
Table 67 shows that T-tests verified this finding by indicating that impact speeds on highways with full or partial access control are significantly different from impact speeds on highways with no access control. 4.4.2 Impact Speed and Angle Models As mentioned above, impact speed and angle have traditionally been believed to be correlated because of the reduction in cornering associated with higher speeds.
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From page 44... ...
However, the Interstate highway classification had shown an acceptable goodness-of-fit test using normal distribution fits and untransformed angle data. Figures 11 and 12 present normal distribution fits to impact speed and square-root impact angle data, respectively, to illustrate how close these estimated distributions are to the raw data.
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From page 45... ...
⎧⎨⎩ ⎛⎝ ⎞⎠= − − − −1 2 1 1 2 12 2πσ σ ρ ρ μ σ exp 2 2 2 + − − − − ⎛ ⎝⎜ ⎞ ⎠⎟ ⎡ ⎣⎢ ⎛⎝ ⎞⎠ ⎛⎝⎜ ⎞ ⎠ y x y y y x x y y μ σ ρ μ σ μ σ ⎟ ⎤⎦⎥ ⎫⎬⎭ ( ) ∫∫ f x y dxdyxy , where: µx = impact speed mean µy = impact angle mean σx = impact speed standard deviation σy = impact angle standard deviation ρ = Pearson's correlation coefficient Two basic assumptions are necessary in order to apply the bivariate normal distribution to model impact and speed data: (1)
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From page 46... ...
When the data was segregated by speed limit range instead of highway classification, two of the four ranges were found to have significant dependency between impact speed and angle. Speed limit ranges of 60–65 and 70–75 mph were found to have p values of 0.0315 and 0.0153, respectively.
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From page 47... ...
47 Angle (degree) Speed (mph)
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From page 48... ...
Figure 13 presents a plot of vehicle orientation distribution from the ran-off-road crash database. Note that less than 40% of crashes were found to have heading angles between –20 and +20 degrees.
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From page 49... ...
For this figure, the data from the current study was limited to access-controlled freeways with speed limits of 70–75 mph. The Cooper data were restricted to divided highways with Figure 13.
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Notice that the 17-22 travel distances are close to those from Cooper and that the differences can be explained by the higher speed limits associated with the current study. Figure 15 illustrates the effects of speed limit by comparing data from the current study collected on access-controlled highways with 55–65 mph speed limits to the Cooper data taken from divided highways with 59–62 mph (95–100 km/h)
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Table 82 shows that there is a relatively strong trend for departure length to increase with higher speed limits. The effects of traffic volume and access control on departure lengths were then explored as shown in Tables 83 and 84, respectively.
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From page 52... ...
to the design of guardrail layouts. Note that the runout length recommendations were based upon the upper tail of encroachDeparture Length Percentile Speed Limit Access Control No.
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From page 53... ...
The 90th percentile departure length for 70–75 mph speed limits with full access control is virtually identical to the recommended guardrail runout length for a 70 mph design speed and high traffic volume. However, the recommended runout lengths for lower traffic volumes appear to drop faster than would be indicated from the 17-22 accident data shown in Table 85.
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From page 54... ...
Recommended guardrail runout lengths for a 50 mph design speed also compare well with departure lengths from roadways with speed limits of 45–50 mph and no access control. Note that the recommended runout lengths are consistently 3 m longer than the measured departure lengths shown in Table 85.
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