"The number of lanes needed for a facility is usually determined during the concept stage of project development. It is usually the number of lanes necessary to accommodate the expected traffic volumes at a level of service determined to be appropriate for the facility." Travel Lanes

The number of lanes needed for a facility is usually determined during the concept stage of project development. It is usually the number of lanes necessary to accommodate the expected traffic volumes at a level of service determined to be appropriate for the facility (see Chapter 4 for a discussion of level of service). The number of lanes can only be added in integer units, i.e., a two-lane highway can be widened to three or four lanes. Each additional lane represents an increase in the traffic-carrying capability of the facility.

Knowing future projected travel demands, the designer, using the analysis procedures in the Highway Capacity Manual, can provide input into the decisionmaking process during project development to determine the appropriate number of travel lanes for the level of service desired. Community input also plays a part in this decision. A community may decide through public involvement that a lower level of service is acceptable for the situation than the level of service normally provided for new construction projects.

In urban and suburban areas, signalized intersections are usually the predominant factor controlling the capacity of the highway or street. There may be more latitude in determining the number of lanes for these types of facilities. For example, a two-lane facility approaching an intersection can be expanded to four lanes (one left turn lane, two through lanes, one right-turn lane) at the intersection itself and then returned to two lanes beyond the intersection. The need to distribute traffic safely will determine the need for any expansion of the approach roadway. The added lanes at the intersection can be in a variety of configurations to serve the travel desires of the traffic.