Caltrans Standard (Mandatory, Advisory, Good Practice)
#1 Lane: 12’ #2 Lane: 12’ #3 Lane/Parking/Shoulder: 20’ minimum (12’ lane w/ 8’ min. shoulder)
Single Left Turn Lane: 12’ Additional Left Turn Lane: 11’
Two-Way Left Turn Lane: 12’ minimum, 14’ preferred.
Desired Conditions Identified by Local Agencies
#1 Lane: 11’ (proposes elimination of left shoulder/shy distance) #2 Lane: 11’ #3 Lane/Parking/Shoulder: 19’ (11’ lane w/ 8’ parking lane or shoulder)
Local agency proposed alternatives include a minimum 19’ #3 Lane/Parking/Shoulder; however, proposed alternatives for the Cities of San Carlos, South San Francisco and San Mateo propose a #3 Lane and shoulder width ranging from 10.5’ to 16’
Single Left Turn Lane: 9.5’ to 11’ Additional Left: NA
Local agencies propose 10’ to 11’ left turn lanes. Proposed alternatives for the City of South San Francisco include 9.5’ left turn lanes. Proposed alternative for the City of San Mateo includes 16’ two-way left turn lane.
Caltrans' Criteria for Design Exception
Requires Fact Sheet Exception to Mandatory Design Standard.
Fact Sheet Exception to Mandatory Design Standard may be submitted for proposed non-standard lane widths. Caltrans will consider 11’ lane widths for through lanes, 10’ widths for single left turn lane and 11’ widths for double left turn lanes. Exceptions to the standard 12’ lane width are justified by right-of-way constraints, environmental issues and accident history. Ideally, exceptions will be justified by authoritative studies and research. Such research and studies should provide safety data for low speed highway facilities which include trucks and buses and preferably with comparable vehicle volumes.
Justification for Design Exception or Design Modification
Justify narrower lanes based on right-of-way constraints, environmental issues and accident history.
Consider narrower lanes to bring other non-standard features to standard width such as narrow parking lane, narrow bike lane, or non-accessible sidewalk.
Local Agency Requirements for Caltrans Review
Collaborate with Caltrans on design exceptions during planning stages.
See checklist for Fact Sheet Exception to Mandatory Design Standard and example Fact Sheet.
See checklist of required studies and supporting data for preparing Fact Sheets.
Justification supported by authoritative studies/research, or documented case studies and accident analyses.
