One way to check an existing or planned signal timing scheme is to calculate the delay experienced by those who are using, or who will use, the intersection. The delay experienced by the average vehicle can be directly related to a level of service (LOS). The LOS categories, which are listed below, contain information about the progression of traffic under the delay conditions that they represent. This allows you, as a designer or evaluator, to visualize and understand the traffic flow conditions surrounding an intersection, even though the intersection might still be on the drawing board.

The first step in the LOS analysis is to calculate the average delay per vehicle for various portions of the intersection. You might be interested in the LOS of an entire approach, or alternatively, you might be interested in the LOS of each individual lane. The equation for the average vehicle delay is given below.

Average Stopped Delay Per Vehicle:

d= [0.38C(1-g/C)2]/[1-(g/C)(X)] + 173X2[(X-1) + [(X-1)2 + (16X/C)]1/2]

Where:
d = Average stopped delay per vehicle for the lane or lane group of interest (sec)
C = cycle length (sec)
g/C = green ratio for the lane or lane group
g = The effective green time for the lane or lane group (sec)
X = V/c ratio for the lane group
V = The actual or design flow rate for the lane or lane group (pcu/hour)
c = Capacity of the lane group (pcu/hour)

This equation predicts the average stopped delay per vehicle by assuming a random arrival pattern for approaching vehicles. The first term of the equation accounts for uniform delay, or the delay that occurs if arrival demand in the lane group is uniformly distributed over time. The second term of the equation accounts for the incremental delay of random arrivals over uniform arrivals, and for the additional delay due to cycle failures.

As was mentioned before, the level of service for signalized intersections is defined in terms of average stopped delay per vehicle. This delay is directly related to the driver's level of discomfort, frustration, fuel consumption, and loss of travel time. The following paragraphs describe the various LOS categories.

Level of Service A - Operations with low delay, or delays of less than 5.0 seconds per vehicle. This LOS is reached when most of the oncoming vehicles enter the signal during the green phase, and the driving conditions are ideal in all other respects as well.

Level of Service B - Operations with delays between 5.1 and 15.0 seconds per vehicle. This LOS implies good progression, with some vehicles arriving during the red phase.

Level of Service C - Operations with delays between 15.1 and 25.0 seconds per vehicle. This LOS witnesses longer cycle lengths and fair progression.

Level of Service D - Operations with delays between 25.1 and 40.0 seconds per vehicle. At this LOS, congestion is noticeable and longer delays may result from a combination of unfavorable progression, long cycle lengths, and high V/c ratios.

Level of Service E - Operations with delay between 40.1 and 60.0 seconds per vehicle. This LOS is considered unacceptable by most drivers. This occurs under over-saturated intersection conditions (V/c ratios over 1.0), and can also be attributed to long cycle lengths and poor progression.

As you can see by now, the LOS illuminates the qualitative aspects of signal operation.