TRAVEL SPEED EFFECTS OF 70 MPH SPEED LIMIT CHANGE

Abstract

An important purpose of speed limits is to improve traffic safety by reducing the risks imposed by the speed choices of drivers. This purpose is achieved in two ways. On one hand, by setting an upper bound, speed limits create a controlling threshold and hence aim to reduce both the probability and severity of crashes. On the other hand, speed limits reduce dispersion in speeds among drivers and thereby reducing potential vehicular conflicts. In addition to this primary reason, speed limits also help improve the efficiency of traffic flow by achieving an orderly flow of traffic [1]. Despite these benefits, establishing an arbitrary value for the speed limit may have a negative impact on traffic flow and safety. Hence, implementing a speed limit is effective only if set within an appropriate range. This range varies from one road to another and is determined based on several factors such as roadway functionality, roadway characteristics, sight distance, and others. As this value shifts from this range, the effectiveness declines. Therefore, in establishing speed limits for a roadway or highway segment, decision makers try to set a reasonable balance between safety (crash risks) and mobility (travel time) [2] [3].Various aspects of speed are used depending on the application. The following terms are commonly used in speed literature and discussions: • Design speed: a selected speed used to determine the various geometric design features of a roadway. • Operating speed: the speed at which vehicles are observed operating during free flow conditions. Free flow speeds are those observed from vehicles whose operations are unconstrained by traffic control devices (e.g., traffic signals) or by other vehicles in the traffic stream. The 85th percentile of the distribution of observed speeds is the most frequently used measure of the operating speed. • Posted speed: the maximum legal speed a vehicle can operate at for a particular location as displayed on regulatory signs. • 85th percentile speed: the speed at or below which 85 percent of vehicles travel. [4] When designing a new road, the characteristics of the facility guide the choice of a design speed. Speed limits are often determined by the driver operating speeds (85th percentile speed – the speed at or below which 85% of all vehicles are observed to travel under free flowing conditions past a monitored point) which in turn affect the timing of traffic signals. Achieving consistency between design speeds, actual driving speeds, and posted speed limits is also taken into consideration [1]. In 1995, the US Congress decided that setting speed limits on the roads in the United States is no longer the responsibility of the federal government. States were allowed to determine and establish their own maximum speed limits. Many states raised their speed limits at that time [3] [5]. Wisconsin State Legislature passed the 2015 Wisconsin Assembly Bill 27, effective May 21, 2015. This bill increased the maximum allowable highway speed limit from 65 mile per hour (mph) to 70 mph on any freeway, including those that are part of the national system of interstate and defense highways and on any section of an expressway that gives priority to through traffic by using interchanges only [6] [7]. As a result, the Wisconsin Department of Transportation (WisDOT) implemented a 70 mph speed limit on most interstate segments. This speed increase affected approximately 726 miles of interstate the week of June 16, 2015 with the exclusion of segments in the Milwaukee area.