Effects of Countdown Timer onthe Type II Dilemma Zones

Very few studies have investigated the distribution variation of the Type II dilemma zone under the condi-tion of with and without countdown timer at intersections,and the conclusions drawn to date are controversial.The objec-tive of this study is to further examine the effect of countdown timers on the Type II dilemma zone through videotaping vehicle movements at an unsignalized intersection.When the signal changes to yellow,the speeds and distances to the stop bar of the first vehicle stopped and the last vehicle passing through are measured and collected.Driving behavior under the two scenarios of with/without countdown timer is analyzed.A binary Logit regression is used to develop the stopping probability model under the two scenarios.The boundary of the type II dilemma zones is determined and the type II dilem-ma zones under the two scenarios are investigated.The results show that stopping probabilities increases when vehicles have longer distances from stop bar and/or with countdown timers;while higher vehicle speeds decrease the stopping like-lihood.The Type II dilemma zones move closer to the stop line when a countdown timer is installed,but its length is still 24.1 m.Therefore,countdown timers will not reduce the range of the Type II dilemma zones.However,in this study, the interaction effects of countdown timers and other variables are not considered during the process of determining the boundaries of the Type II dilemma zones,which will probably lead to underestimating the effectiveness of countdown tim-ers.