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Capacity and LOS: Theory and Concepts

Basic Freeway Section and Ideal Freeway Conditions

A basic freeway section is a segment where there are no interruptions to the flow of traffic. Interruptions to traffic flow occur when vehicles enter or leave the freeway. Therefore, a basic freeway section is one where on or off ramps are not present for at least 1500 feet upstream and downstream of the section.

In addition to uninterrupted conditions, the "ideal" basic freeway section is defined as having the following characteristics:

    • Each lane is 12 feet wide.
    • There is 6 feet of clearance between the outside and the inside edges of the freeway and the nearest obstruction that would distract or influence a motorist.
    • All vehicles are passenger cars (no trucks, buses, or recreational vehicles).
    • Ten or more lanes (in urban areas only).
    • Interchanges are spaced every 2 miles or more.
    • The drivers are regular and familiar users of the freeway section.
    • The terrain is level, with grades no greater than 2%.

Together, these conditions represent the "highest" (ideal) type of freeway section, which is one with a free-flow speed of 70 mph or higher and a capacity of 2400 passenger cars per hour per lane (pcphpl).