Keywords: saturation flow rate, cumulative curves, signalized intersections, traffic signals, traffic.

In the developing countries, drivers and law enforcement officers, behave differently than their counterparts in the industrialized countries. The drivers are more aggressive and less compliant to the traffic laws and the officers are not so serious in enforcing the traffic laws. Such driver's behaviour is demonstrated by their lack of lane discipline, and queuing behaviour that doesn't follow the first come first serve rule. Consequently, traffic parameters such as saturation flow rate, control delay, level of service among others which play an important rule in determining the performance of signalized intersections, are significantly affected [1].

Saturation flow rate represents the capacity of the lanes serving traffic movements through the approaches of signalized intersections. The highway capacity manual (HCM) defines saturation flow rate as the maximum service rate of flow, which can be accommodated by the lane group under the prevailing roadway and traffic conditions, assuming that the green phase was always available to the lane group [2]. The period of saturation flow which has a reasonably uniform distributed headways among the discharged vehicles, extends from the time the rear axle of the fourth queued vehicle passes the stop line or a reference point and the time the rear axle of last vehicle queued, passes the same point. The saturation flow rate is then computed as the reciprocal of the average headway of the discharged vehicles during this period and expressed in vphg (vehicles per hour of green) [3].

The method for the analysis of signalized intersections as presented in Chapter 16 of the HCM is rather complicated, and requires a very detailed traffic, geometric and environmental information about the intersection to be analyzed [4]. It uses a base saturation flow rate of 1,900 pcphgpl which must be adjusted by using adjustment factors for prevailing roadway and traffic conditions such as: lane width, left turns, right turns, heavy vehicles, grades, parking, parking blockage, area type, bus blockage, and left turn blockage.

The adjustment factors included in the HCM were established according to research that was conducted in circumstances that exist in the industrialized countries which are different than the existing conditions in the developing countries. The application of such parameters needs calibration prior to their use in the developed countries, which may render them useless. Moreover, the method does not account for the different behaviour of drivers in such countries.

The HCM includes an alternative method in Appendix H to Chapter 16 to measure the prevailing saturation flow rate directly in the field. This measurement can be used directly in operational analysis of signalized intersections as a substitute for the computations of the saturation flow module. This procedure is practical to be used in the field for one or two lane approaches, but for four lane approaches, it becomes extremely impractical, especially in the developing countries where there is a significant interaction among the traffic lane. This interaction is due to the frequent lane changing by some inconsiderate drivers who cut off other drivers to make their time to the stop line shorter. Consequently, there is a necessity to substitute for the use of the first method, which is complex and needs detailed traffic, geometric and environmental information, as well as the alternative direct field measurement method, which is impractical for such circumstances. The substitution method should be easier and suit conditions that exist in the developing countries.

This study tries to meet this necessity by estimating the aggregate approach saturation flow rate using a method that utilizes the cumulative discharge curves and takes into consideration the impact of driver behaviour on the estimated values of saturation flow rate. An aggregate saturation flow rate was estimated for a traffic data that was collected for a typical four legs signalized intersection in the city of Makkah in Saudi Arabia during the morning rush hour. The estimated approach saturation flow rate was 6.8% and 8.1% lower than the calculated values using the HCM third module, and the average field measurement methods, respectively.

1

Saif, M.A., "Development of a Procedure to Estimate Control Delay", Proceedings of The Tenth International Conference on Civil, Structural and Environmental Engineering Computing, Topping, B.H.V., (Editor), Civil-Comp Press, Stirling, United Kingdom, Paper 92, 2005. doi:10.4203/ccp.81.92

2

Transportation Research Board (TRB), "Highway Capacity Manual", National Research Council, Washington D.C. 2000.

3

May, A.D., "Traffic Flow Fundamentals". Prentice-Hall, Inc, Englewood Cliffs, New Jersey, USA, 1990.

4

Banks, J.H., "Introduction to Transportation Engineering", Second International Edition, McGraw-Hill, New York, 2004.

purchase the full-text of this paper (price £20)

go to the previous paper
go to the next paper
return to the table of contents
return to the book description
purchase this book (price £105 +P&P)