The number of vehicles on our highways increases every year, and transportation engineers are often faced with the challenge of designing modifications to existing facilities that will service the increased demand. As part of this work, the engineers must evaluate the capacity of the existing and proposed systems. In addition, engineers are often required to justify the expense of modifying or adding facilities by looking at the current and potential levels of service.

Capacity and Level of Service (LOS) are closely related and can be easily confused. To help clarify the difference between the two, imagine a phone booth that contains ten people. The phone booth obviously has a capacity of ten or more people, but it’s likely that the level of service (quality of service) would be unanimously unacceptable. Capacity is a measure of the demand that a highway can potentially service, while level of service (LOS) is a measure of the highway’s operating conditions under a given demand.

Traffic engineers use capacity and level of service analyses to:

• Determine the number and width of lanes needed for new facilities or for expanding existing facilities.
• Assess service levels and operational characteristics of existing facilities that are being considered for upgrading.
• Identify traffic and roadway changes needed for new developments.
• Provide base values for determining changes in fuel consumption, air pollutant emissions, road-user costs, and noise associated with proposed roadway changes.

Capacity and level of service (LOS) are fundamental concepts that are used repeatedly in professional practice. Because of their obvious importance, this chapter is designed to introduce the undergraduate engineering student to capacity and level of service (LOS).

To begin exploring Capacity and Level of Service, click on one of the links to the left.