SHOULDER UPGRADING ALTERNATIVES TO IMPROVE OPERATIONAL CHARACTERISTICS OF TWO-LANE HIGHWAYS

A research project was undertaken to develop upgrading warrants for use in determining when to add paved shoulders to rural two-lane roadways or when to convert two-lane roadways with paved shoulders into four-lane undivided roadways by low-cost treatments such as remarking the shoulder to indicate that it is a travel lane. The latter treatment is known as a poor-boy highway in Texas. The findings of one portion of the research--the effects of paved shoulders on vehicle operating characteristics--are described. Field studies were performed at 18 sites around Texas for three types of highways: (a) two-lane roadways without shoulders, (b) two-lane roadways with shoulders, and (c) four-lane, undivided roadways. Operational characteristics were recorded for more than 21,000 vehicles. Data were gathered on speed, platooning, shoulder use, and vehicle type. The findings indicate that operational benefits derived from a full-width paved shoulder increase as traffic volumes increase. These benefits are minimal at low and moderate volumes, but they become significant at volumes greater than about 200 vehicles/h. Above this volume, paved shoulders appear to increase the average speed on the roadway by at least 10 percent. They also limit the number of vehicles in platoons to less than 20 percent. No more than 5 percent of all vehicles used the shoulder at any of the sites. Conversion of the shoulder to an additional travel lane offers no apparent operational benefits until the volume reaches 150 vehicles/h. On higher-volume roads, this modification could be expected to cause average roadway speeds to increase by approximately 5 percent and limit platooning to 5 percent. Significantly, such a conversion results in more than two-thirds of the traffic using the outside (shoulder) lane. (Author)