Field measurement of delay at traffic signals is a costly and cumbersome process, and the use of analytical models to estimate delay is, therefore, of interest to the traffic engineer. A model originally developed by Webster has gained widespread use and acceptance in the estimation of delay at pretimed signals where signal timing remains constant from cycle to cycle. The original version of this model has been modified for application to traffic-actuated signals where signal timing is determined on the basis of vehicle presence information received from detectors in the roadway. This paper describes the modifications to Webster's model, which consist primarily of the substitution of values in the second (random arrival) term based on maximum cycle length rather than on optimal or average cycle lengths. The delay calculations that result from the modified version are compared with the values for pretimed operation based on the original model. Both versions of the model are compared with a simulation model and found to produce satisfactory approximations. Delay under traffic-actuated control is lower than delay under pretimed control. The difference depends on the degree of saturation of the approach lanes. The maximum difference is observed at 75 percent saturation. No difference is observed at very low saturation levels because very little delay accrues under these conditions. The difference also approaches zero at very high saturation levels because the actuated controller becomes constrained by the maximum interval timer to operate in a pretimed mode. /Author/

Media Info

  • Media Type: Digital/other
  • Features: Figures; References;
  • Pagination: pp 17-21
  • Monograph Title: Evaluation of transportation operational improvements
  • Serial:

Subject/Index Terms

Filing Info

  • Accession Number: 00170559
  • Record Type: Publication
  • ISBN: 0309026555
  • Files: TRIS, TRB
  • Created Date: Apr 12 1978 12:00AM