Crossing an Arterial through an Offset Intersection: Bicycle-Only Center-Turn Lane
Source: City of Portland, Oregon
Background
The North-South 40's Bikeway is a 7.6-mile bicycle corridor about 2.5 miles from Portland's downtown core. Developed in 1999, the bikeway runs the entire breadth of Portland from north to south, connecting residential neighborhoods to five commercial districts, six parks, and ten schools, and intersecting ten perpendicular bikeways. It comprises 5.6 miles of bicycle boulevards, 1.8 miles of bicycle lanes, and about 500 feet of off-street path.
Portland stripes bicycle lanes on roads with average daily traffic volumes of 3,000 or greater. Bicycle boulevards are low volume streets that generally work well for bicycling. The city typically improves arterial crossings, alters the stop sign pattern, and occasionally diverts automotive traffic to make them work better.
A minor arterial with an average daily traffic of about 10,000, SE Stark Street, intersects a segment of the bikeway on SE 41st Avenue. The junction is complicated by a 115-foot offset of 41st as it crosses Stark. North and south approaches are stopped with stop signs. The standard set of crossing treatments were considered but posed significant drawbacks for this project. The only effective option would have been a median refuge, which would have prohibited some turning movements from Stark to 41st.
Countermeasures
The City of Portland considered two options—crossing making first a right turn and then a left turn; or using the next street to cross, making first a left turn and then a right turn. Doing the latter would require only striping the receiving bicycle lanes on the cross street. That was rejected in favor of the right-turn first scenario because to make the left turn first would necessitate crossing both lanes of cross traffic at once, rather than crossing one lane at a time, as is done when making the right turn first.
In the end, it was decided to stripe a bicycle-only center-turn lane. This two-way, 10-foot lane provides a refuge for bicyclists who cross Stark by essentially executing first a right-turn onto Stark, and then a left-turn back onto the bikeway.
Evaluation and Results
While no formal evaluation has yet been completed, the feedback from bicyclists has been positive and the intersection continues to function as intended. This treatment likely provides operational and safety benefits for bicyclists by reducing overall crossing distance and providing space for bicyclists to wait for a sufficient gap in traffic before completing their crossing.
This treatment for offset intersections was included in Portland's report, Bikeway Facility Design: Survey of Best Practices. The report is part of Portland Bicycle Plan for 2030. As one of the recommended design practices, bicycle-only center-turn lanes will be considered in the current 20's and 50's North South Bikeway projects as a possible treatment for offset intersections. Other cities that have implemented similar design features are Seattle, WA; Tucson, AZ; and De Bilt in the Netherlands.
Conclusions and Recommendations
This treatment successfully addressed three criteria: it offered a refuge for crossing bicyclists and allowed them to cross one direction of traffic at a time; it maintained all automotive turning movements; and it provided an inexpensive solution to this crossing that left more available funding for conventional treatments at other intersections on the bikeway.
The One Year Progress Report on immediate actions for the Portland Bicycle Plan for 2030 included an action item about continuing to experiment with, and evaluate, new facility types identified in Bikeway Facility Design: Survey of Best Practices. This will involve working with Portland State University and the National Association of City Transportation Officials to study and develop guidelines for new facility types including bicycle-only center-turn lanes.
Costs and Funding
Costs for thermoplastic paint to make the bike markings were minimal. The project was implemented as part of a larger plan, so there is no break-out cost for this treatment.
References
Geller, R., and T. Borkowitz. Report on Immediate Actions; One Year Progress Report. Portland Bureau of Transportation. 2011. Avalaible: http://www.portlandoregon.gov/transportation/article/345420
Bikeway Facility Design: Survey of Best Practices. Portland Bureau of Transportation, Design Working Group. 2010. Available: http://www.portlandoregon.gov/transportation/article/334689
Contact
Roger Geller
Bicycle Coordinator
City of Portland Office of Transportation
1120 SW 5th Avenue, Room 800
Portland, OR 97204
