Urban Operations Research
Introduction to Queueing Theory and Its Applications
Requests for service very often find some or all of the units associated with some urban service system busy. The performance of these systems on such occasions (i.e., under conditions of some "stress") plays a major role in shaping citizens' perceptions with regard to the level of service offered.
That some deterioration in the level of service will occur during periods of intensive activity is clear. For instance, a fire alarm may occur at an instant when all fire companies stationed at the nearest fire houses have already been dispatched to another alarm elsewhere in a city. To the extent, then, that the response time to the new fire will be longer under these circumstances (because of the need to dispatch fire engines from remote fire stations), the perceived level of service will be lower. Similarly, a police dispatcher will often postpone service to medium-priority calls for police assistance during periods when nearly all police cars in a district are busy. In doing so, the dispatcher preserves the ability of the police to respond immediately to top priority calls while maintaining some level of police "visibility" through the patrolling activities of non busy cars. Under normal conditions such medium priority calls would have received prompt service.
Link to material: http://web.mit.edu/urban_or_book/www/book/chapter4/contents4.html