Since the imposition of the 55mph 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 Southwest Research Institute ( 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 automo biles 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 at 15 % . Small automobiles seen 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 .