Design Traffic and Level of Service

Level of service or LOS is a qualitative term describing the density of traffic, and relating travel speeds, delays, and other measures to performance.

Design Traffic and Level of Service

Designers also have a choice for the level of traffic for which they and stakeholders wish to design; and the quality of traffic service to be provided. Design traffic is usually expressed as an hourly volume (referred to as design hour volume or DHV), generally derived from a long range travel forecast for the project.

The DHV or design year traffic is not often thought of as a choice; indeed, in many cases the traffic forecast or projection is provided to the project team by an outside stakeholder or agency such as an MPO. However, designers and stakeholders should not lose sight of the fact that they can choose to accommodate either many or fewer hours of the year in which the DHV will occur as shown in Exhibit F-9 (CSD_130).

Exhibit F-9 (CSD_130)
Guidelines for Selecting Design Hour Volume
A related design choice is the design level of service (LOS). LOS is a qualitative term describing the density of traffic, and relating travel speeds, delays, and other measures to performance. LOS is defined differently for the range of highway types and operating conditions, including freeways (mainline, ramps, and weaving sections), two-lane highways, intersections, arterial highways, transit facilities, and pedestrian facilities. LOS ranges from A to F, with LOS E generally representing operation at the practical capacity of the highway (or highway segment). The Highway Capacity Manual represents best practices in terms of procedures for defining, calculating, and designing for LOS. AASHTO recommended LOS targets are shown in Exhibit F-10 (CSD_155).

Exhibit F-10 (CSD_155)
Design Levels of Service Recommended by AASHTO
An early application of the principle of choice in designing for traffic is the reconstruction of the North Central Expressway (U.S. 75) undertaken by the City of Dallas and Texas DOT. Initial planning studies conducted in the mid 1980s concluded that the available right-of-way and overall corridor context would not allow for accommodating the theoretical demand forecast by the MPO. A level of traffic, and implied level of service E was established as the design basis following extensive discussions with stakeholders. This decision greatly shaped the design solution.

Designers have choices regarding the LOS to which a given design should be targeted. The choice should reflect the problem being addressed and the context. AASHTO provides some guidance in this area (see Exhibit F-10 (CSD_155)), but note that the AASHTO values are considered just guidance, and not mandates, requirements, or a design issue requiring a design exception. Indeed, the AASHTO policy contains considerable discussion on the issue of designing for congestion (which would be LOS E), in recognition that in some cases LOS E is all that is practically feasible. Most states also have specific guidance in their policies and manuals regarding design LOS. As the design LOS applies to the design year traffic, it will generally be as high as is considered practical.

The choice of an appropriate LOS should be based on the project purpose, and on judgements regarding future traffic increases, and the consequences of under-designing. Note that the FHWA does not consider LOS as a design issue requiring a design exception if published guidance is not met.

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