Incident Dispatching, Clearance and Delay

Problem

Dispatchers for mobile emergency crews, such as police and freeway service patrol trucks, frequently must make a choice between sending the closest available crew or waiting for one that's closer, but busy.

Method

This paper builds a simplified model (i.e., one emergency vehicle per incident, one incident at a time, evenly spaced highway interchanges) to help formulate the best dispatch strategies to use when incidents cause delay. The model considers different crew deployment methods: individual, non-overlapping beats; and rolling beats, first with fixed spacing between emergency vehicles and then with random spacing between vehicles based on Poisson process locations.

The model's purpose is to determine the relationships between system parameters (such as spacing between interchanges and the time needed to maneuver through an interchange) and system performance. Our objective is to minimize traffic delay at incidents. Our model considers the distance of response vehicles to the incident, whether the vehicle is on the same side of the highway as the incident, the position of interchanges, and the presence of congestion.

Findings

A response system using individual, set beats causes longer waits but is more stable than a system using rolling beats.

Any robust dispatching strategy must look for a balance between certainty in response time and stability in the system. Limiting response distance and allowing some calls to queue is essential to stability. Waiting for the closer, but busy, vehicle can reduce service time and keep the dispatch system stable at higher use levels, but it can also add uncertainty about the length of expected delay. As more variation is introduced, it becomes more advantageous to go ahead and dispatch the available but more distant vehicle, rather than wait.