Module 7 : Traffic Signal Design

Lecture 36 : Special Requirement in Traffic Signal

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	Interval design There are two intervals, namely the change interval and clearance interval, normally provided in a traffic signal. Change interval The change interval or yellow time is provided after green time for movement. The purpose is to warn a driver approaching the intersection during the end of a green time about the coming of a red signal. They normally have a value of 3 to 6 seconds. The design consideration is that a driver approaching the intersection with design speed should be able to stop at the stop line of the intersection before the start of red time. Institute of transportation engineers (ITE) has recommended a methodology for computing the appropriate length of change interval which is as follows: $$Y=t+\frac{v}{2(gn+a))}$$ (1) where $t$ is the reaction time (about 1.0 sec), $v$ is the velocity of the approaching vehicles, $g$ is the acceleration due to gravity (9.8 m/sec2), $n$ is the grade of the approach in decimals and $a$ is the deceleration of Change interval can also be approximately computed as y = SSD/v, where SSD is the stopping sight distance and v is the speed of the vehicle. The clearance interval is provided after yellow interval and as mentioned earlier, it is used to clear off the vehicles in the intersection. Clearance interval is optional in a signal design. It depends on the geometry of the intersection. If the intersection is small, then there is no need of clearance interval whereas for very large intersections, it may be provided. Clearance interval The clearance interval or all-red will facilitate a vehicle just crossed the stop line at the turn of red to clear the intersection without being collided by a vehicle from the next phase. ITE recommends the following policy for the design of all read time, given as $$R_{AR}== \left\{ \begin{array}{lll} \frac{w+L}{v}&\mbox{if no ped... ...f pedestrian crossing} \\ \frac{P+L}{v}&\mbox{if protected} \end{array}\right.$$ (2) where $w$ is the width of the intersection from stop line to the farthest conflicting traffic, $L$ is the length of the vehicle (about 6 m), $v$ is the speed of the vehicle, and $P$ is the width of the intersection from STOP line to the farthest conflicting pedestrian cross-walk.