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Problem 4: Analysis

We have now completed a review of the operation of Alternate Route 7 during the PM peak period. The operation of this facility is typical of many urban freeways during peak periods. At the beginning of the peak, the facility operates at acceptable levels of service, and all demand is served during the first 15-minute time period. During the second time period, a queue begins to form as the demand exceeds the capacity where the facility drops from three lanes to two. This is a classic freeway bottleneck condition. The queue extends from the bottleneck point between sections 5 and 6 (where the lane drop occurs) upstream through section 5 and into part of section 4. The bottleneck, and the resulting queue, delays vehicles that entered the system during time period 2 to the next time period. The queue clears during time period 3, and the freeway is back to good operation during time period 4.

The table below provides a summary of some of the key data for the four time periods that we have reviewed.

Parameter

 Time period 1 Time period 2 Time period 3 Time period 4
Density (veh/mi/lane)
Queueing?
Mainline travel time (min/veh)
Mainline traffic delay (min/veh)
Mainline speed (mi/hr)
Level of service range		 16.4
No
5.88
0.06
54.42
A - C		 36.9
Yes
7.15
1.33
44.15
C - F		 11.3
Yes
6.64
0.82
47.03
A - E		 7.2
No
5.91
0.09
53.96
A - B

These summary data provide several interesting insights, at a more system level, on the performance of the freeway facility.

  • Even in time period 1, when there is no queuing, there is delay. How can this be true? Recall that delay is the time that a driver spends traveling at less than preferred speed. The average mainline speed is less than 55 mph (54.42 mph), so there is some, though minimal, delay. The delay increases during time period 2 to nearly 1.5 minutes per vehicle, and the average speed drops to 44.15 mph.

  • These system measures do provide a broad perspective on the performance of the freeway facility and are therefore valuable aids to analysts and decision makers. However, to understand the specific causes of delay or queuing, we must always look at the details of the facility performance, checking the data for each section to understand the causes of poor performance.

What are the implications of these results? Do we need to continue with further analysis of this freeway facility? When you are ready, proceed to the next page.

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