Determination of Optimum Offset Pattern in Traffic Networks
1970 Volume 6 Issue 6 Pages 523-530
Details
1970 Volume 6 Issue 6 Pages 523-530
Lately, the traffic congestion in road networks has become a serious problem as cars increase. The enormous cumulative waiting time at signals is not only a loss of time but a cause of air pollution. A systematic signal control may decrease the loss.
Considering the signal control of traffic, there are three parameters, that is, the cycle time, the cycle split (ratio of green time to cycle time) and the offset (phase difference between two signals). Concerning the traffic networks, the offset control is the most effective and important of the three.
It is known that there is an optimum offset between two neighbouring signals which minimizes the waiting time of two-way traffic flow. However, if a traffic network system is very large and complicated, it is very difficult to find the combination of all offsets, that is, the offset pattern, which minimizes the total waiting time for the whole network. The purpose of this paper is to find the optimum offset pattern.
The author shows such traffic network systems can be optimized by using the technique of dynamic programming, if the waiting time of each pair of intersections is given as a function of offset. Moreover, he applies linear programming to this problem by approximating the waiting time by two linear functions and finds easily the optimum solutions of more complicated systems using a macroscopic model of traffic flow.
The optimum offset patterns of some network examples obtained by D.P. and L.P. are compared with the results of macro-model simulation. In applying the optimum control obtained by D.P., the total waiting time is decreased by 40 to 70 percent compared with that when there is no control (random offset), and by 10 to 50 percent compared with that when the simultaneous signal systems or the alternate signal systems, or the favoring systems are adopted. Similarly, the offset patterns obtained by L.P. decrease the total waiting time by 15 to 60 persent with no control and by 3 to 35 percent with control.
