Since its enactment in 1974, the National Maximum Speed Limit (NmSL) of 55 miles per hour has remained a controversial law. Despite the enforcement efforts, recent studies have found that approximately 20 percent of all American motorists exceed the 55 m.p.h. limit. However, legislation still requires individual states to enforce the law or else forfeit millions of dollars in federal highway aid. This study examined three questions: (a) is there a preferred speed that drivers find comfortable during travel, (b) how does it compare with the national speed limit, and (c) what is its relationship to vehicle design and driver characteristics? Taken into account were factors that made the study representative of national highway driving. For example, each of the 100 participants was selected on the basis of demographics one finds in the driving population (i.e., age category, sex, driving experience and vehicle type). Furthermore, each participant used his own vehicle to drive a highway route of 60 miles, which included typical highway settings (two-lane and four-lane, divided and un-divided) with average traffic volumes. So that the speed was truly a matter of preference, the instrument panels were covered and the only true indication of velocity was the highway environment, other vehicles, or the vestibular sense. An accelerometer was used to record both the vibration and sound levels of the sampled vehicles. The results showed that the subjects in the study preferred a comfortable driving speed which exceeded the national maximum speed limit. More specifically, in regard to the 55 m.p.h. speed limit, the study revealed: (a) only 37% of the drivers preferred to drive 55 m.p.h. or less, (b) 34% of the drivers preferred to exceed 55 m.p.h. by 5 m.p.h., and (c) 29% of the sample established a comfortable speed of 60 m.p.h. or greater. The results of this study are indicative of the fact that highway drivers prefer to drive at a speed in excess of 55 m.p.h. For this reason it is recommended that consideration be given to raising the national speed limit to 60 m.p.h., thereby bringing it into line with an average speed that the majority of drivers prefer. This law would help to lower the variance of highway speeds and thus reduce the number of accidents and fatalities on our national highways. This investigation also demonstrated significant differences in preferred driving speeds between age categories. The younger drivers in this study (i.e., 16–29) preferred speeds well above the national speed limit of 55 m.p.h. Such results have important implications for the State Licensing Bureau.
Operational and Safety Evaluation of Freeways with Posted Minimum Speed Limit
