A probabilistic assessment of dilemma and option zone problems

Author

Jennifer Harper Ogle

Date of Award

12-1996

Degree Type

Thesis

Degree Name

Master of Science

Major

Civil Engineering

Major Professor

Lee D. Han

Committee Members

Stephen Richards, Fred Wegmann

Abstract

The Institute of Transportation Engineers currently recommends that the phase-change interval of a traffic signal be set using a constant approach speed equal to the 85^th percentile approach speed, a constant deceleration of 10 ft/sec², and a constant reaction time of 1 second. Previous studies have shown that drivers do not all travel at the same speed and decelerate at the same rate, nor do they possess the same reaction time. In fact, the practice of using the 85^th percentile approach speed in itself denounces the idea that all drivers would approach an intersection at a single speed. Thus, it would appear that the current practice may be inappropriate. A probabilistic approach is developed here to estimate the effects of variance in approach speed during different periods of congestion and free-flow traffic conditions. These effects are measured by determining the magnitude of the dilemma or option zone problems for any given situation. Using previously published values of reaction time, deceleration rates and acceleration rates, and approach speed distributions, a spreadsheet calculation procedure was developed to estimate the magnitude of the dilemma/option zone problems. The following approach yields logical results regarding the probability of being caught in a dilemma zone or option zone for given distributions of approach speed at different levels of congestion or free-flow traffic conditions. As a result, the approach will enhance the practice of designing the phase-change interval to minimize these zones. Some transportation engineers might dispute the use of the dilemma zone as a measure of effectiveness, by reason of the 1962 change in the Uniform Vehicle Code definition of the yellow signal indication. This change allows for vehicles to enter during the yellow interval and clear during the red. There is, however, no rule for providing an all-red clearance interval. This means that the driver may clear during an all-red clearance interval or during the beginning of green for the opposing traffic stream. The legality issue of clearing during the opposing green is covered by the Uniform Vehicle Code in a statement of the green indication. This statement says 'drivers facing green must yield to vehicles legally in the intersection at the time of such indication.' Hence, drivers would never be faced with the dilemma of not being able to stop before entering the intersection or not being able to clear before the onset of red. In an informal survey of 100 people in Knoxville, TN, where the vehicle code follows the one stated above, 100 % were unknowing of at least one implication of the signalized intersection laws as stated in the Uniform Vehicle Code. According to those surveyed, it seems quite clear that they may be encountering dilemma zones because of their lack of understanding of the current laws. With the possibility of this being public perception nationwide, it seems imperative that transportation professionals assess the magnitude of problems that these drivers may encounter. For this reason, the use of the dilemma zone magnitude, as a measure of effectiveness, is warranted.

Recommended Citation

Ogle, Jennifer Harper, "A probabilistic assessment of dilemma and option zone problems. " Master's Thesis, University of Tennessee, 1996.
https://trace.tennessee.edu/utk_gradthes/10853