Capacity

Abstract

How much traffic can a facility carry? This is one of the fundamental questions designers and traffic engineers have been asking since highways have been constructed. The term “capacity” has been used to quantify the traffic-carrying ability of transportation facilities. The value of capacity is used when designing or rehabilitating highway facilities to determine their geometric design characteristics such as the desirable number of lanes, it is used to design the traffic signalization schemes of intersections and arterial streets, it is used in evaluating whether an existing facility can handle the traffic demand expected in the future, and it is also used in the operations and management of traffic control systems (ramp metering algorithms, congestion pricing algorithms, signal control optimization, incident management, etc.).

Problems

• 1. Conduct a literature review to identify previous studies that quantify capacity-related measures. Provide numerical values of capacity estimates for various locations around the USA and abroad.

• 2. Using Fig. 4.2, and assuming that capacity is defined to occur when the probability of breakdown is 0.25, determine the following: (a) What is the capacity of the ramp if the demand from the mainline is 4,500 vph? (b) What is the capacity of the mainline freeway if the metered ramp supplies 1,380 vph? Can you relate the numbers in the graph to the capacities provided in the HCM 2010 method for freeways?

• 3. Two “critical” freeway-ramp junctions (1 and 2) experience breakdown events independently. Assume that the ramps are metered, but there are no data available regarding ramp metering rates. Assume also that based on freeway data collected at the two segments, the breakdown volume is Weibull distributed with parameters:

  • (a) $A1=18.36\kern1em \beta 1=2,269$

  • (b) $A2=17.85\kern1em \beta 2=2,105$

  Find the expected value E(q) and the standard deviation (σ) of the estimated Weibull breakdown volume. For any time interval i, what is the probability of breakdown at each ramp junction if the mainline volume exceeds 2,000 veh/h/ln?

• 4. Conduct a literature review on the two-capacity phenomenon. Should our analysis tools provide two different capacity values, one before breakdown and one after?

• 5. Your State Department of Transportation is interested in estimating what the implications of such a finding are. Assuming that the per lane capacity is 2,300 pcph before the breakdown, and the maximum throughput drop is 10 %, calculate the following: (a) What is the difference in throughput under the two scenarios over an hour for a three-lane facility? (b) If the expected demand of the facility is 2,500 pcph over the peak hour, what is the expected difference in maximum queue length, total delay, and average delay per vehicle at the end of the hour? (c) What is the percent difference in the average delay per vehicle between the two scenarios?