According to the Department of Transportation Federal Highway Administration website, the minimum lane width for an interstate highway is 12-feet.
The Context-Sensitive Solutions in Designing Major Urban Thoroughfares for Walkable Communities, produced by the ITE (Institute of Transportation Engineering), states that appropriate lane widths for arterial and collector streets are between 10-ft and 11-ft.
I would like to use Union Avenue in Colorado Springs as an example of how a bicycle lane could be included. The segment of Union Avenue that I feel would be used most often is adjacent to the Velodrome at Memorial Park, which is used by the cyclists from the Olympic Training Center’s cyclists. At this location, the current street configuration includes four lanes of through traffic and a center turn lane. The street dimension is approximately 58-feet, as measured on the Springgov.com GIS mapping website. If the lane widths were to decrease to a dimension of 10.5-feet and the center turn lane were decreased to 10-feet, 6-feet would remain for a bicycle lane on the Memorial Park side of the street. We have instances similar to this all over our city, as do other cities.
I have completed entitlement work in small towns where the minimum lane width is in excess of 13-feet for a local street with an intended speed limit of 25 mph. Can you guess where the police in those towns will be conducting a lot of their speed traps? It won’t be on the older, original streets of the town.