Khaled

The HCM recommends 120 sec, and may reach 150 sec for extreme cases. I myself have seen 180 sec in Michigan for an intersection, which later converted to a grade separation (for obvious reasons). The higher the traffic volume the more likely the cycle length requirement will increase. The reason for restricting the cycle length to a maximum value is to limit vehicular delays. Depending on the methodology used to estimate delay, this can be proven mathematically.

Best

Dear Dr. Khaled,

Thank you for you response. I tried to find these values in the HCM 2010 but I couldn't. can you please provide me with page number in which these values are stated. Furthermore, I tried to make signal timing optimization by HCS 2000  and  I found that the maximum allowable cycle length is 230s.

Cycle Length DeterminationThe following excerpt was taken from section 12-306.3 of the 1995 Idaho Transportation Department Traffic Manual.

Cycle Length

Cycle length is composed of the total signal time to serve all of the signal phases including the green time plus any change interval. Longer cycles will accommodate more vehicles per hour but that will also produce higher average delays.

The best way is to use the shortest practical cycle length that will serve the traffic demand. Vehicles at a signal installation do not instantaneously enter the intersection. Early studies by Greenshields found that the first vehicle had a starting delay of 3.7 seconds to enter the intersection with subsequent vehicles requiring an average of 2.1 seconds each. Generally, vehicles will pass over an approach detector with a headway of 2 to 2.5 seconds. For general calculation purposes, an average time of 2.5 seconds per vehicle to enter the intersection is a conservative value. This value can be used to estimate signal timing for planning purposes.

The cycle length includes the green time plus the vehicle signal change interval for each phase totaled to include all signal phases. A number of methods have been used to determine cycle lengths as outlined in the Highway Capacity Manual, ITE Manual on Traffic Signal Design, and ITE Transportation and Traffic Engineering Handbook. Webster provided the basic empirical formula that would minimize intersection delay as follows:

C = (1.5*L + 5)/(1.0 - SYi))

Where:

C = optimum cycle length in seconds adjusted usually to the next highest 5 second interval. Cycle lengths in the range of 0.75C to 1.5C do not significantly increase delay.

L = Unusable time per cycle in seconds usually taken as a sum of the vehicle signal change intervals.

SYi = critical lane volume each phase/saturation flow

The saturation flow will be between 1500 and 1800 vehicles per hour. Refer to Highway Capacity Manual. The "Y" value should be computed for each phase and totaled to arrive at SYi for all phases.

Note: The traffic volumes used should be the predicted volumes at time of signal turn-on. The volumes should also be the peak hour or peak fifteen-minute period for the cycle determination.

When the cycle length has been determined the vehicle signal changes are deducted giving the total cycle green time which can be proportioned to each signal phase on the basis of critical lane volumes. The individual signal phase times are then the proportioned time plus the vehicle change interval on each phase.

To ensure that critical lane volumes are adequately served, a capacity check should be computed for each green interval.

There is generally no maximum value of the cycle length since the objective of signal design is to minimise delays to all approach legs at the intersection. The longer the cycle length the more the delay. Since the approach legs do not have the same demand volume, the cycle length is proportioned according to the demand from the approach legs. However, some approach legs could experience very high flows that could warrant long green phases for them. This could result in other approach legs not getting enough green phases to discharge traffic. Drivers from such approaches could become impatient and behave unsafely. Hence the cycle length is restricted by certain road jurisdictions. This ensures that traffic is not unduly held at the intersection for too long and every approach has a chance to move in a short while. The Webster approach  is the simplest method usually employed.

Dear Khaled,

Maximum allowable is generally recommended as 120-150 secs. But in some cases, it can be extended as 250 secs. The restriction is related to delays of vehicles. If you use very long times for cycle length, the delays of vehicles at the intersection increase. So the optimum cycle formula that gives the minimum delay was developed.

I hope this answer can be helpful...

Dr. Y. Sazi Murat 

In practice it should rarely be greater than 150 s! with 120 s being the norm.  Usually the optimum point is around 100 - 120 s if you do the delay vs. time plot for the particular intersection.  Long cycle times frustrate drivers and encourage red light running.  Usually  more efficiency can be gained from reducing the number of phases and also possibly double cycling (repeating) a phase.  If the demand still requires cycle time above 150 you should use physical interventions. i.e more lanes at the stop line to reduce stacking queues, and also more downstream lanes to get the starting platoon clear of the intersection and not block-back from a downstream merge shock wave.   if it still is above 150 s then grade separate.

Many values have been indicated for the maximum length of a cycle, in general as a report of the maximum lengths surveyed in various situations. Actually the length of the cycle is the result of an optimization problem, which depends on the number of phases and the widths of the legs of the intersection. The phase for a leg is constituted by a time interval in which cars cross the STOP line of the intersection (effective green), by a red time and by a time interval used  to clear the intersection, which increases with the width of the leg that is crossed. This time interval is lost time, because during it all cars are still at the STOP lines of all legs. Given the number of phases and the width of the legs, the ratio between lost time and effective green decreases as the length of the cycle increases. However, as we can see from the various formulae proposed for the computation of the waiting time (e.g Webster, HCM, etc), the latter increases with the cycle length. Thus the optimum cycle is the result of a trade-off between the need of reducing the lost time compared to the effective green and that of decreasing the waiting time.

it is mentioned in HCM 2010.But, it depends on the maximum allowable delay to keep certain LOS. in HCM 2010, 6 level of services is defined according to delay in Exhibit 18-4. So, based on the flow characteristics and saturation properties of the intersection, and based on E level of service, you can reversely calculate the maximum cycle length allowable. Generally, most researcher approximate it between 200 to 220 sec.

Dear Khaled,

You know its related to the delay in the intersection approaches..short cycle time leads to high delay value also the same for long cycle time..there is some optimum cycle length value ..these value range from 60 sec to 140 sec. but the maximum value does not mentioned in the HCM..but the lower value is mentioned to allow for pedestrian crossing..regards

It may be too late to answer this now. an isolated intersection normally has no pedestrian traffic. If one uses pedestrian crossing requirement then green times can be added.