4.3.4 Highway Capacity

Highway capacity is used to express the maximum hourly rate at which persons or vehicles can reasonably be expected to traverse a uniform section of a lane or roadway during a given period under roadway and traffic conditions. Highway capacity encompasses broader relations between highway characteristics and conditions, traffic composition and flow patterns, and the relative degree of congestion at various traffic volumes.
To determine the capacity for a particular roadway design, the designer should refer to the
HCM
for guidance. The
HCM
is used as the basic reference for performing operational analysis that may be appropriate depending on the circumstances and includes procedures for analyzing the LOS for all modes of travel.
Roadway capacity analysis serves three general purposes including:
  • Transportation planning studies:
     used to assess the adequacy or sufficiency of existing roadway networks to service current traffic along with estimation of the future traffic growth that may exceed current capacity;
  • Roadway design:
     used to select the roadway type and to determine dimensions such as the number and types of lanes and minimum lengths for weaving sections; and
  • Traffic operational analyses:
     used to identify bottleneck locations of the roadway and to estimate operational improvements that may result from prospective traffic control measures or spot alterations in the roadway geometry.
While traffic volumes are the main contribution to the serviceability of a facility, roadway design features can also affect operating conditions. These roadway factors can be described as the following:
  • Alignment
     for traffic traveling at any given speed, the better the roadway alignment the more traffic it can carry. It follows that congestions will generally be perceived at lower volumes if the design speed is low. The roadway should be subdivided into sections of consistent geometric design characteristics for analysis using the HCM techniques. A single limiting curve or steep grade in otherwise gentle alignment will be identified as the critical feature limiting roadway capacity.
  • Weaving Sections:
     roadway segments where the pattern of traffic entering and leaving at contiguous points of access results in vehicle paths crossing each other. Where the distance in which the crossing is accomplished is relatively short in relation to the volume of weaving traffic, operations within the roadway section will be congested. Some reduction in operating efficiency through weaving sections can be tolerated by roadway users if the reduction is minor and the occurrence is not high.
  • Ramp Terminals:
     these features can adversely influence operating conditions of freeways if the demand for their use is excessive or if their design is deficient. When congestion occurs at the freeway ramp junctions, typically drivers will avoid this lane and shift to the inside lanes causing congestion to the through lanes. The loss in efficiency is a function of the volume of traffic entering or leaving ramps, and the geometric layout of the terminals.
  • Mixture of Vehicles:
     traffic streams are usually composed of a mixture of vehicles including passenger cars, trucks, buses, and occasionally recreational vehicles and bicycles. The combination of these vehicles creates a flow rate that is not uniform throughout the hour, day, season, or year.
  • Peak Hour Factor:
     the factor used to convert the rate of flow during the highest 15-minute period to the total hourly volume is the peak hour factor (PHF). It is the ratio of the total hourly volume to the number of vehicles during the highest 15-minute period multiplied by 4.
4.3.4.1 Level of Service (LOS)
The HCM defines the quality of traffic service provided by specific roadway facilities under specific traffic demands by means of a LOS. The LOS characterizes the operating conditions on the facility in terms of traffic performance measures related to speed and travel time, freedom to maneuver, traffic interruptions, and comfort and convenience. The levels of service range from LOS A (least congested) to LOS F (most congested). The HCM procedures for estimating the traffic operation LOS should be used for all contexts. Designers should provide the highest LOS practical and consistent with the project functional and context classification.
Chapters 5 through 8
discuss the target LOS based on functional class.
Table 4-2: Level of Service
Level of Service
General Operating Conditions
A
Free Flow
B
Reasonably Free Flow
C
Stable Flow
D
Approaching Unstable Flow
E
Unstable Flow
F
Forced Breakdown Flow