Short note: fixed and operational delay

Delay is the extra travel time lost compared with free travel; it is a measure of the level of service of an intersection or road section. Total delay has two parts.

Fixed delay

• Caused by traffic control devices or physical features and not by other vehicles. It occurs even when there is no interference from other traffic.
• Examples: stopping at a traffic signal, stop sign, railway level crossing, toll plaza or check post.
• It is largely independent of traffic volume; at a signal it depends mainly on the cycle length and green split.

Operational delay

• Caused by interference of one component of traffic with another, so it grows with traffic volume.
• Examples: side friction from pedestrians and parked or stopping vehicles, slow vehicles, turning vehicles, merging and weaving, and queuing behind accidents.

$$\text{Total delay} = \text{Fixed delay} + \text{Operational delay}$$

Delay is commonly measured as stopped-time delay (time the vehicle is standing still) and travel-time delay (difference between the actual and the ideal travel time); both fixed and operational causes contribute to these. They are measured by the floating-car method or by a stopped-delay (point sample) study and are used to evaluate the level of service and the benefit of improvements.