Feasibility Study of Retrofitting Concrete Median Barriers
Edward Post, Patrick McCoy
Since the imposition of the 55 mph speed limit in 1974, accident data on the concrete median barriers (CMB) in California showed that the fatal + injury accident rates are increasing. Also, the California data showed that 7.7% of the 1,515 reported accidents in 1978 and 9.9% of the 1,796 reported accidents in 1979 resulted in vehicle rollover. In comparison accident data summarized by SwRI showed that 3.9% of the 180 reported accidents prior to 1974 resulted in rollovers and mountings. Little or no information on the number by type of vehicle involved in rollovers was reported. However, the findings in this study indicate that this significant increase in rollovers since 1974 is undoubtedly due to (1) an increase in travel speeds, and (2) an increase in the number of small automobiles in the traffic stream. It is predicted that the rollover rate will continue to increase in the future and by 1985 it could be as high as 15%. Small automobiles seem to have a greater tendency to rollover on the CMB than the earlier standard size automobiles, for which the CMB was designed, because of their shorter wheel track widths and much lower roll-moment-of-inertia. The proposed retrofit unit concept for improving the rollover performance characteristics of the standard New Jersey CMB was investigated in this feasibility study. Basically, the retrofit unit consists of reverse sloped surfaces to suppress vehicle uplift and rollover under impact angles greater than 10 deg. The retrofit unit would be of precast concrete construction and anchored to the CMB by rebar dowels and epoxy. The findings in this feasibility study indicate that the retrofit unit has the potential of being a cost-effective improvement alternative on (1) rural interstate highways with 30 ft medians and carrying an ADT greater than 66,000 vpd, and (2) urban interstate highways with 16 ft medians and carrying an ADT greater than 117,000 vpd. These findings were based on an assumed retrofit unit cost of $10/ft, a compact automobile split of 50%, and accident societal costs published by the National Safety Council. As the compact automobile splits increase above 50% in the near future, the above breakeven ADT volumes would decrease. Based on the findings in this study, it is recommended that the potential effectiveness of the retrofit unit in suppressing vehicle uplift and rollover be confirmed by conducting full-scale vehicle crash tests. To aid in the selection of the test vehicles and impact conditions, it is recommended that the California accident records for 1978 and 1979, in which 8.8% if the 3,311 reported accidents resulted in rollovers, be examined manually.
Concrete Median Barriers, Roadside Safety Appurtenances