Chapter 13 - Traffic Forecasting and Analysis Concepts

13.2. Freeway Traffic Analysis and Design

13.2.2. Capacity Analysis and Level of Service

TRB defines capacity as the maximum hourly rate at which persons or vehicles reasonably can be expected to traverse a point or uniform segment of a lane or roadway during a given period under prevailing roadway, traffic, and control conditions; adding that “Capacity analysis is a set of procedures for estimating the traffic-carrying ability of facilities over a range of defined operational conditions" (2000, p. 2-1).

Service flow rates are similar because they define the flow rates that be accommodated while still maintaining a given level of service.

There are numerous factors that affect capacity and LOS:

• base conditions 
• prevailing roadway conditions (including geometric and other elements)
• prevailing traffic conditions, which also account for vehicle type (e.g. heavy vehicles) and distribution of vehicles

For design LOS for GDOT roadways, refer to Chapter 6, Tables 6.1 through 6.4 of this Manual.

Traffic Flow Characteristics

Traffic flow on a freeway can be highly varied depending on the conditions constraining flow at upstream and downstream bottleneck locations. Bottlenecks can be created by ramp merge and weaving segments, lane drops, maintenance and construction activities, accidents, and objects in the roadway. An incident does not have to block a travel lane to create a bottleneck. For example, disabled vehicles in the median or on the shoulder can influence traffic flow within the freeway lanes.

Freeway research has resulted in a better understanding of the characteristics of freeway flow relative to the influence of upstream and downstream bottlenecks. Freeway traffic flow can be categorized into three flow types:  (1) under-saturated, (2) queue discharge, and (3) oversaturated. Each flow type is defined within general speed-flow-density ranges, and each represents different conditions on the freeway.

Under-saturated flow represents traffic flow that is unaffected by upstream or downstream conditions. This regime is generally defined within a speed range of 55 to 75 mph at low to moderate flow rates and a range of 40 to 60 mph at high flow rates. 

Queue discharge flow represents traffic flow that has just passed through a bottleneck and is accelerating back to the free-flow speed of the freeway. Queue discharge flow is characterized by relatively stable flow as long as the effects of another bottleneck downstream are not present. This flow type is generally defined within a narrow range of 2,000 to 2,300 passenger cars, per hour, per lane (pcphpl), with speeds typically ranging from 35 mph up to the free-flow speed of the freeway segment. Lower speeds are typically observed immediately downstream of the bottleneck. Depending on horizontal and vertical alignments, queue discharge flow usually accelerates back to the free-flow speed of the facility within 0.5 to 1 mile downstream from the bottleneck. Studies suggest that the queue discharge flow rate from the bottleneck is lower than the maximum flows observed before breakdown. A typical value for this drop in flow rate is approximately 5 percent.

Oversaturated flow represents traffic flow that is influenced by the effects of a downstream bottleneck. Traffic flow in the congested regime can vary over a broad range of flows and speeds depending on the severity of the bottleneck. Queues may extend several thousand feet upstream of the bottleneck. Freeway queues differ from queues at intersections in that they are not static or ‘standing.’  On freeways, vehicles move slowly through a queue, with periods of stopping and movement.

Speed-Flow and Density-Flow Relationships

The free-flow speed of passenger cars (mph) on freeways is relatively insensitive to flow rate of passenger cars per hour per lane (pcphpl) in the low to moderate range (0 pcphpl to 1,200 pcphpl). Studies have shown that passenger cars operating at a free-flow speed of 70 mph maintain the operating speed for flows up to 1,300 pcphpl  For lower free-flow speed, the region over which speed is insensitive to flow extends to higher flow rates. In general terms, the lower the flow rate, the higher free-flow speed of the vehicle. Similarly, the higher the flow rate, the higher the density, which is measured in passenger car per mile per lane (pc/mi/ln).

Refer to the current TRB Highway Capacity Manual Chapter 13, Freeway Concepts, for a detailed discussion and exhibits specific to Speed-Flow and Density-Flow Relationships and factors that affect free-flow speed.

Passenger-Car Equivalents

The concept of vehicle equivalents is based on freeway conditions in which the presence of heavy vehicles, including trucks, buses, and recreational vehicles, creates less than base operating conditions. These diminished operating conditions include longer and more frequent gaps of excessive length both in front of and behind heavy vehicles, the speed of vehicles in adjacent lanes, and the physical space taken up by a large vehicle (typically two to three times greater than a passenger car). To allow for these lesser travel conditions and ensure the method for freeway capacity is based on a consistent measure of flow, each heavy vehicle is converted to a passenger-car equivalent. The conversion results in a single value for flow rate in terms of passenger cars per hour per lane (pcphpl). The conversion factor depends on the proportion of heavy vehicles in the traffic stream and the length as well as the severity of the roadway grade.

Driver Population

Studies have shown that non-commuter driver populations display different, less aggressive characteristics than regular commuters. For recreational traffic, capacities have been observed to be as much as 10 to 15 percent lower than for commuter traffic traveling on the same segment.