Question

a)A route within an urban setting has three signalized intersections in series and at maximum intervals of 50m from one intersection to the next. During peak periods, there is backflow of traffic from one intersection to the previous. As a Traffic Engineer, what cost effective measure will you propose to alleviate this problem and how will you achieve it? It should be noted that physical improvements such as relocation of intersection, lane widening, channelization, etc. cannot be considered as options.

b) Most of the traffic signal systems used in Ghana are pre-timed signals. There are instances where phases with high traffic volumes have green times lower than phases with low traffic volumes. What could account for this signal timing deficiency and what can be done to rectify it? (AN,5)

Answer 1

a) When you have three signals which are spaced 50 m from each other, then you will have a platoon of vehicles which depart from the 1st signal when it turns green and then will show up at the 2nd signal. So in order to have an effective signal system, what you need to do is to have all the three signals as actuated coordinated and controlled by one single controller and provide correct offsets

Suppose the travel speed is 40mph (17.8816 meter/sec), then the time taken to travel from signal one to signal two = 2.7961 seconds

So this is the offset for the 2nd signal i.e 2.7961 seconds

The offset for the third signal will be 2 * 2.7961 = 5.5923 seconds

So in order to have good platoon progression and to ensure that vehicles which are discharged from 1st signal continue flowing through the 2nd signal, we need to do the following

a) Make sure the start of the green time for Signal two is offset and made to start 2.7961 seconds after the green time for signal one starts

b) Make sure the start of green time for Signal three is offset by 5.5923 seconds from start of green time for signal one

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Question b) Typically phases with higher traffic volumes should have greater green times than those with lower traffic volumes. We can re-calculate the signal times for all the junctions and allocate correct green times which will help in traffic progression.

As an alternative if we can change the signals from pre-timed to actuated signals, then what we can do is allocate most of the green to the major movements and allocate green to minor movements ONLY WHEN THEY ARE ACTUATED by the presence of any vehicle on the minor approach.

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