Traffic signal design of isolated intersections for mixed traffic operation

Abstract

The control of traffic in cities of developing countries such as Bangladesh has long been dependent on the use of fixed time traffic signals. The reasons are low capital cost, ease of installation and maintenance and, possibly, that they are technologically more appropriate in terms of hardware and software. Such dependence demands a more critical calculation of signal setting and hence good information on the prevailing traffic. Some important variables for signal design are saturation flow, lost times, PCU values etc. Methods of measuring and predicting these signal design parameters have long been developed in the western world. " These well known procedures and measurement techniques are essentially applicable for homogeneous nature of traffic stream which is typically car dominated with vehicles moving in clearly defined lanes. Some vehicle types in developing countries are not dissimilar to those of the developed world, but the traffic operational conditions in a developing country like Bangladesh is quite different. These different traffic operation conditions are featured as heterogeneous vehicle mix i.e. both motarised and non-motorised vehicles comprise traffic stream, difference in driving behaviour and the lack of lane discipline in particular. Attempts to apply the signal design procedures and methods of measuring design parameters that are developed based on homogeneous traffic condition to the non-lane based mixed traffic operations do not always result in desirable success. There is, thus, a need for developing most appropriate signal design procedure and measurement techniques of the parameters involved to accommodate local prevailing traffic conditions. In the light of this, a systematic signal design guidelines have been proposed to help local traffic engineers in signal design. Video recording and as well as manual method were used to collect data for customising different lane based signal design parameters for local condition. In addition, using these customised procedure and parameters, a computer software MIXSIG ( SIGnal design for MIXed traffic operations) has been developed as an aid for the design and evaluation of signalised intersections. The salient features of this software are: • adaptation of different approaches for the measurement of saturation flow values • consideration of variable PCU (passenger Car Unit) values for each vehicle type instead of unique value • inclusion of red violation behaviour of drivers in calculations of initial and final lost times • consideration of a special all red period to allow non-motorised vehicles to clear up junction • consideration of forced"gap acceptance behaviour during the right tuming manoeuvre MIXSIG is a user friendly and an easy-to-use package. It employs a flexible graphical input interface and its output includes signal timing results and phase diagram for different types of intersections. It is expected that the proposed guidelines and the software will enable the field engineers to design signal methodically and thereby will contribute to alleviate the ever increasing traffic congestion in Bangladesh to some extent.