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Clear Zone for Roadways with Design Speed of 70 mph

Question
State WI
Description Text

We are looking to increase our posted speed of the rural freeways to 70 mph.  We have a project that is looking to use a design speed of 75 mph.   I was asked what should be the clear zone for a 75 mph design speed. 

I believe that some of the work Dr. Sicking put together for the NCHRP Report 665 may be able to provide guidance.

Keywords
Other Keywords clear zone
Date April 25, 2012


Response
Response

The clear zone adjacent to high speed roadways was originally determined from lateral encroachment data collected adjacent to high speed test tracks at General Motors.  Every ran-off-roaad event was identified and investigated to determine the vehicle trajectory after leaving the roadway.  The distribution of lateral travel distances was developed from these accident investigations and the national clear zone distance for high speed highways was set equal to the 70th percentile lateral encroachment distance. 

This same approach can be used to estimate the appropriate clear zone distances for high speed highways using data from NCHRP Report 665. This study collected more than 800 vehicle trajectories from single-vehicle, ran-off-road crashes on high speed roadways.  Further, the crash sampling method produced a large bias toward severe crashes.  Thus, even though approximately half of these crashes involved impacts with fixed objects which may tend to shorten the lateral travel distances, the large bias toward more serious crashes should produce the opposite effect.  Thus, the data from NCHRP Project 665 is believed to be the best source of vehicle trajectory data currently available.

                Unfortunately, the number of crashes collected from 75 mph highways was somewhat limited. When lateral encroachment data from controlled access highways with 70 & 75 mph speed limits is examined, the 70th percentile lateral encroachment was found to be 10.5 m or 34.5 ft.   This value closely matches the Roadside Design Guide recommendation of 32-35 ft for 70 mph highways.   Historically, encroachment data has been extrapolated to higher speed facilities by incorporating 80th percentile encroachment distances.  The 80th percentile encroachment distance from the curve below was found to be 13 m or approximately 43 ft.

                The appropriateness of  using this approach to extrapolate encroachment distances to higher speed limit facilities was then evaluated by using data from 65 mph highways to estimate the appropriate clear zone at 70 mph.   As shown in the figure below, the estimated clear zone width for 65 and 70 mph roadways was found to be 8.3 and 10.4 m respectively.  The close correlation between the two estimates for 70  mph roadways and the correlation with the RDG provide strong support for the method used to estimate appropriate clear zone for 75 mph highways.  


Date September 28, 2012
Attachment Estimating Clear Zone Distances from Lateral Encroachment Distance Distribution.pdf


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