This study consisted of two phases. The first phase measured the lateral time gap used by drivers entering a major road stream of traffic from a side street controlled by a stop sign. The driver either turns or crosses the path of travel of the approaching vehicles. The second phase examined the frontal time gap used by drivers making a left turn at a traffic control light that does not have a left turn arrow. The first phase study results reflect 5,306 observations over three locations in Des Moines, Cedar Rapids, Davenport, Dubuque, Council Bluffs, and Sioux City. These cities have populations between 50,000 and 200,000. These results are as follows: drivers on the minor roadway allowed themselves the largest lateral gap for left turns, followed by a smaller gap for passing straight through the intersection, and even less time for making a right turn; there was no difference in the lateral time gap used by male and female drivers; there was a longer delay time for drivers on the minor roadway when the intersection had a stop sign rather than a traffic control light; drivers on the minor roadway selected a larger time gap when the vehicle on the major roadway approaches from either the left or right side, as contrasted with vehicles approaching from both sides; drivers used the greatest time gap during the morning hours, a smaller time gap during the noon hours, and an even smaller gap in the early evening hours; the speed limit (30-35 mph) on the major roadway did not affect the lateral time gap of the drivers on the minor roadway; and traffic volume on the major roadway did influence the lateral gap selection of drivers on the minor roadway. Exceptions to these findings are noted in the report. The phase two survey was conducted at signalized, traffic controlled intersections in two urban cities in Iowa. The study subjects were making a left turn at a traffic control light that did not have a left turn arrow. The timing of the frontal time gap began as the left turning vehicle began to move into the turn and the oncoming vehicle reached a designated reference point in the intersection area. The results are as follows: drivers in the morning hours selected a longer frontal time gap than at noon and early evening; the higher the speed limit (45 mph or higher) on the major roadway for the turning vehicle, the longer the frontal time gap used; there was no significant difference in frontal time gap by sex, age, vehicle type, and roadway environments; and traffic volume did not significantly affect the frontal gap. (A)
Samenvatting