U.S. Department of Transportation
Federal Highway Administration
1200 New Jersey Avenue, SE
Washington, DC 20590
202-366-4000
This is the decision matrix for converting from either four or five lanes to a three-lane configuration.
The process begins with determining the average daily traffic (ADT) on the roadway in question. If the average daily traffic is greater than 25,000, conversion is not feasible. If it is less than 10,000, conversion is feasible. If the road's ADT is between 10,000 and 16,000 with 1/4 to 1/2 mile signal spacing, a key intersection analysis is required. If the ADT is above 16,000 but below 25,000 or if there is less than or equal to 1/8 mile spacing, a corridor analysis will be required.
When a key intersection analysis is required, if there are more than 700 vehicles per hour per day or more than 200 vehicles per hour turning left, then a Synchro model should be used. If there are less than 700 vehicles per hour per day or less than 200 vehicles per hour turning left, then no model is required. When using the Synchro model, if the result of the model is a level of service (LOS) and critical approach less than or equal to E, proceed to T.O. and S.O. manager approval. If the result is LOS F or critical approach F, the design should be modified and remodeled until those results are improved. Once T.O. and SO manager approval has been given, the next step is to proceed with the community process.
When a corridor analysis is required, a Synchro model should be used. If the results indicate less than 30 percent travel time change, if the corridor LOS is D or better, and if the LOS is less than or equal to E at critical approaches, proceed to TO and SO Manager approvals and commence community process. If, however, the result is a 30 percent increase in travel time or an increase of 2 LOS levels, the design must be modified until the acceptable parameters required for TO and SO manager approvals are met before commencing with the community process.
