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52 Table 86. Departure lengths excluding barrier impacts. Speed Access No. of Departure Length Percentile Limit Control Cases 90% 85% 80% 75% 70% 50% 70-75 Full 137 111 101.6 88.7 77.2 67 52.5 55-65 None 263 67.9 55.6 49 46 43 31.8 45-50 None 201 61.8 48.4 41.8 36.8 32.6 24.7 sufficient data to provide departure lengths for both full and There was also a concern that rigid objects may have an no access control. effect on longitudinal travel distances. This concern is based on the assumption that, for most crashes involving a rigid obstacle, impacting vehicles are brought to a premature stop. 4.5.2 Screened Data In this situation, the length the vehicle travels along the The data shown in Table 85 provides measures of the roadside would be artificially reduced. This effect was again length of vehicle departures for several speed limit and access explored by removing crashes involving rigid obstacles from control categories. Although this table represents the actual the data set and re-tabulating the data as shown in Table 87. travel distances associated with serious injury and fatal Again, the effects of removing rigid obstacle crashes from the crashes, the data may be distorted by the placement of longi- database were extremely minor. The average change in depar- tudinal barriers. Barriers placed adjacent to the travelway are ture length between Tables 86 and 87 was found to be less designed to redirect vehicles away from roadside obstacles than 0.5%. Based upon the minor differences in Tables 85, 86, and toward the travelway. Thus, longitudinal barriers are and 87, it can be concluded that the upper tails of the road- likely to reduce the length of travel along the roadside and the side departure length distributions from the 17-22 database departure length data shown in Table 85 may be artificially are not significantly affected by the presence of roadside bar- shortened. The effects of longitudinal barriers on the length riers or rigid obstacles. Thus it is recommended that Table 85 of roadside travel were investigated by removing all crashes be used in the evaluation of guardrail runout length calcula- involving barrier impacts. The data shown in Table 85 was then tion procedures. adjusted by excluding all crashes involving barrier impacts and is presented in Table 86. Note that the number of cases in 4.6 Significance for Guardrail the 5565 mph, full access-control category was reduced to Runout Length the point that the tail of the distribution could not be reliably determined. Further, eliminating barrier impacts increased As mentioned previously, guardrail length-of-need is longitudinal travel distance values for access control freeway determined based upon the design runout length. This length by an average of 2% and decreased lengths for roadways with- is used to identify locations along the roadway in advance out access control by approximately 1%. The minor differences of a roadside object where barriers must begin to be effective. between Tables 85 and 86 appear to indicate that longitudinal Table 88 shows the recommended runout lengths contained barriers do not produce a significant reduction in the dis- in the 2006 AASHTO Roadside Design Guide. As mentioned tances that vehicles travel along the roadway during ran-off- above these values are based on the H&K encroachment data road events. This finding may indicate that, for most impacts, (7). Table 89 presents runout length recommendations longitudinal barriers do not redirect cars back onto the road- from a 1996 study that applied Cooper's data (33) to the way, but rather allow impacting vehicles to rub along the face design of guardrail layouts. Note that the runout length rec- of the barrier. ommendations were based upon the upper tail of encroach- Table 87. Departure lengths excluding barrier and rigid obstacle impacts. Speed Access No. of Departure Length Percentile Limit Control Cases 90% 85% 80% 75% 70% 50% 70-75 Full 136 111.5 101.7 88.9 78.2 67.4 53 55-65 None 262 67.9 55.7 49 46 43 31.6 45-50 None 196 62.3 48 41.7 36 32.2 24.6

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53 Table 88. Runout length recommendations from the RDG. Traffic Volume ADT 6000 2000-6000 800-2000 <800 Runout Runout Runout Runout Design Speed (mph) Length (m) Length (m) Length (m) Length (m) 70 146 134 122 110 60 122 112 101 91 50 98 89 81 73 40 73 67 61 55 30 49 45 41 37 ment length distributions from H&K and Cooper. For per year. Therefore, there is a penalty for placing too much Table 88, the top row of runout lengths were obtained from guardrail adjacent to the roadway and excessive guardrail the 85th, 80th, 75th, and 70th percentile runout lengths length is likely to produce greater numbers of serious in- from the H&K study. Because the Cooper study contained no juries and fatalities than would be associated with shorter highways with 70 mph speed limits, the top row of Table 89 installations. was obtained by extrapolating the 90th, 85th, 80th, and Note that findings from the 17-22 data compare much 75th percentile encroachment lengths from the divided better to guardrail length guidelines developed from Cooper. highways with 5962 mph speed limits included in the The 90th percentile departure length for 7075 mph speed Cooper study. limits with full access control is virtually identical to the rec- When the data from the 17-22 study shown in Table 85 ommended guardrail runout length for a 70 mph design is compared with RDG runout length guidelines, it is clear speed and high traffic volume. However, the recommended that existing guardrail design procedures greatly overesti- runout lengths for lower traffic volumes appear to drop faster mate guardrail lengths. Note the 90th percentile departure than would be indicated from the 17-22 accident data shown length shown in Table 85. Note that the recommended runout in Table 85. However, the recommended lengths do match length for high traffic volumes with a 70 mph design speed up well with the 80th, 75th, and 70th percentile departure is approximately one-third greater than the 90th percentile length from Table 85. Recall that the original guardrail length departure length found along access-controlled freeways guidelines were developed based on the 85th through 70th with speed limits from 70 to 75 mph. The difference between encroachment lengths from the H&K data. The approach the 17-22 departure lengths and the H&Kbased runout was shifted slightly to utilize the 90th through 75th percentile lengths increases further until it reaches 46% for traffic vol- encroachment length when Cooper data was utilized in place umes of less than 800 average daily traffic (ADT), which were of the H&K study. This adjustment was implemented because intended to correlate with the 70th percentile encroachment Cooper's data did not include any highways with speed lim- length. Thus, 17-22 data indicates that the guardrail length its greater than 62 mph. When the entire history of guardrail recommendations contained in the RDG grossly overstate length determination is considered, the guardrail runout guardrail need. It is important to note that guardrail is a length recommendations for a 70 mph design speed shown roadside hazard that produces approximately 1200 fatalities in Table 89 are found to compare very well with the 17-22 Table 89. Runout length recommendations from Wolford & Sicking (36). Traffic Volume ADT 10,000 5,000-10,000 1,000-5,000 <1,000 Runout Runout Runout Runout Design Speed (mph) Length (m) Length (m) Length (m) Length (m) 70 110 91 79 67 60 79 64 55 49 50 64 52 46 40 40 49 40 34 30 30 34 27 24 21

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54 departure length distribution for access-controlled freeways speed also compare well with departure lengths from roadways with 7075 mph speed limits. with speed limits of 4550 mph and no access control. Note Note that for design speeds of 60 mph, guardrail runout that the recommended runout lengths are consistently 3 m lengths shown in Table 89 appear to be midway between the longer than the measured departure lengths shown in Table 85. full access control and no access control data for 5565 mph In summary, with the exception of highways with a design speed limits. If it is assumed that fully access-controlled free- speed of 60 mph and full access control, guardrail length rec- ways are designed to a 70 mph or higher design speed, guard- ommendations based on Cooper's data compare surprisingly rail runout length recommendations shown in Table 89 can well with departure length data described herein. Therefore, be considered to be conservative. However, if fully access- it is recommended that AASHTO consider adding a recom- controlled roadways utilize a 60 mph design speed, the rec- mendation that guardrails placed along fully access-controlled ommended guardrail lengths should probably be extended. freeways should be designed for 70 mph, regardless of the Recommended guardrail runout lengths for a 50 mph design actual design speed.