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 Signal Timing Design: Example Problems

 
Critical Movement or Lane

As a transportation engineer about to embark on the cycle length and green split calculations, you need to find the critical lane for each phase of a two-phase signal cycle. In this example problem we will only focus on one phase. The approaches that are serviced in this phase will have two lanes, one servicing left-turns and straight-through traffic, and the other servicing right-turns and straight-through traffic. The design flow rates and saturation flow rates for each lane are given below.

Lane Description Design Flow Rate Saturation Flow Rate
North-bound  L,S 600 pcu/hr 1200 pcu/hr
North-bound  R,S 500 pcu/hr 1700 pcu/hr
South-bound  L,S 450 pcu/hr 1330 pcu/hr
South-bound  R,S 720 pcu/hr 1600 pcu/hr

Which lane is the critical lane for this phase, and what is the critical flow ratio for this phase?

[Solution Shown Below]

 

 

 

 

 

 

 

 

 

 

 

 

Solution

The critical lane is the lane that requires the most time to service its queue. It can be found by locating the lane with the highest flow ratio (V/s). Simply calculate the flow ratio for each lane by dividing the design flow rate by the saturation flow rate. Then find the lane with the largest flow ratio.

Lane Description Flow Ratio
North-bound  L,S 0.5
North-bound  R,S 0.294
South-bound  L,S 0.338
South-bound  R,S 0.45

It looks like the north-bound left-turn and straight-through lane is the critical lane for this phase. The critical flow ratio is just the flow ratio for the critical lane (0.5).