Capacity of a freeway segment

Freeway capacity is defined as:

the maximum sustained 15-min flow rate, expressed in passenger cars per hour per lane, that can be accommodated by a uniform freeway segment under prevailing traffic and roadway conditions in one direction of flow.

Capacity analysis is based on freeway segments with uniform traffic and roadway conditions. If any of the prevailing conditions change significantly, the capacity of the segment and its operating conditions change as well. Therefore, each uniform segment should be analysed separately.

Factors affecting Capacity

Roadway conditions

Roadway conditions include geometric and other elements. In some cases, these influence the capacity of a road; in others, they can affect a performance measure such as speed, but not the capacity or maximum flow rate of the facility. Roadway factors include the following:

Number of lanes, Number of lanes decided for basic freeway is five or more than five but if number of lanes is less than five then capacity of freeway is reduced.
Lane widths, If the lane width is less than the specified lane width for basic freeway segment, i.e 3.6m then capacity is reduced because traffic flow tends to be restricted.
Shoulder widths and lateral clearances, shoulder width and lateral clearance influences the capacity of freeway. When lane widths are less than 3.65 m, drivers are forced to travel closer to one another laterally than they would normally desire. Drivers tend to compensate for this by reducing their travel speed. The effect of restricted lateral clearance is similar. When objects are located too close to the edge of the median and roadside lanes, drivers in these lanes will shy away from them, positioning themselves further from the lane edge hence capacity is reduced.
Design speed, freeway is designed for free flow speed around 120 km per hour ,if some vehicle is moving less than the design speed then capacity of freeway.
Grades: Effect of grade depends on both the length and slope of the grade.Traffic operations significantly affected when grades of 3% or more are longer than one quarter miles and when grades are less than 3% and longer than ½ mile.The effect of heavy vehicles on such grades is much greater.

Traffic conditions

Traffic conditions that influence capacities and service levels include vehicle type and lane or directional distribution.

Vehicle type

The entry of heavy vehicles - that is, vehicles other than passenger cars (a category that includes small trucks and vans) - into the traffic stream affects the number of vehicles that can be served. Heavy vehicles are vehicles that have more than four tires touching the pavement. Trucks, buses, and recreational vehicles (RVs) are the three groups of heavy vehicles.

They are larger than passenger cars and occupy more roadway space; and
They have poorer operating capabilities than passenger cars, particularly with respect to acceleration, deceleration, and the ability to maintain speed on upgrades.

Directional and Lane Distribution

In addition to the distribution of vehicle types, two other traffic characteristics affect capacity and level of service: directional distribution and lane distribution. Each direction of the facility usually is designed to accommodate the peak flow rate in the peak direction. Typically, morning peak traffic occurs in one direction and evening peak traffic occurs in the opposite direction. Lane distribution also is a factor on multi lane facilities. Typically, the shoulder lane carries less traffic than other lanes.

Control conditions

For interrupted-flow facilities, the control of the time for movement of specific traffic flows is critical to capacity and level of service. The most critical type of control is the traffic signal. The type of control in use, signal phasing, allocation of green time, cycle length, and the relationship with adjacent control measures affect operations. Stop signs and yield signs also affect capacity, but in a less deterministic way. A Impact of control conditions traffic signal designates times when each movement is permitted; however, a stop sign at a two-way stop-controlled intersection only designates the right-of-way to the major street. The capacity of minor approaches depends on traffic conditions on the major street. An all-way stop control forces drivers to stop and enter the intersection in rotation. Capacity and operational characteristics can vary widely, depending on the traffic demands on the various approaches.