American Institute of Certified Planners (AICP) Practice Exam

At what Level of Service do intersections become unstable, leading to short queues?

1. Level of Service C

2. Level of Service D

3. Level of Service E

4. Level of Service F

The correct answer is: Level of Service D

At Level of Service D, intersections typically experience increased levels of congestion, but they are still operational and can handle traffic effectively. However, this is the point at which the stability of the intersection begins to decline. At this level, traffic volumes often approach capacity, leading to conditions where vehicles start to queue more frequently, resulting in short queues. The characteristics of Level of Service D indicate that while drivers may still find their wait times acceptable, the intersection becomes less efficient due to the higher volume of traffic. As demand approaches the threshold of maximum capacity, the potential for instability increases, leading to the formation of queues that may fluctuate based on the traffic flow. Understanding that queues of vehicles can lead to varying delays and frustration for drivers, it is crucial to recognize that this level represents a critical transition from acceptable operations to unstable conditions. Thus, Level of Service D serves as a key indicator for traffic planners to anticipate and mitigate congestion before it escalates further into more severe levels of service such as E and F, where conditions become much less manageable.