Header photo: Seattle’s first protected intersection. Credit: Seattle DOT

In January, NACTO published the third edition of Urban Bikeway Design Guide, its first update in a decade. The new edition goes beyond a kit of parts to a more complete how-to manual for designing safe, bikeable, inviting streets. Soon, we’ll make the Urban Bikeway Design Guide available for free on our website. In the meantime, we’re posting excerpts each week here on our blog. Can’t wait until the Guide is on our website? Buy it from Island Press, and use code UBDG3 for 20% off.

Today’s excerpt is from Section 4.3: Signalized Intersections.

Signal phasing and timing are core tools for better intersection design. Trade-offs between comfort and convenience are present in all signal operations. Operational analysis informs, but does not predict, how to phase a signalized intersection. Intersection and corridor signal timing analysis, the existing risks and operational issues at an intersection, and an understanding of how people using the street will respond to signals are all important factors in operations decisions.

Along bikeways, signal plans should:

Optimize operations for people biking, walking, and taking transit. Analyze signals as part of a network rather than as isolated intersections. Prioritize safe operations while minimizing bike and pedestrian delay and maintaining desired transit throughput. Eliminate any excess green time for motor vehicle traffic to reduce opportunities for speeding. 

Supplement signal operations with geometric features. Intersections with relatively long motor vehicle queues should feature more physical separation of bikeways, including bikeway setbacks. Buses may need queue jump lanes to maintain route reliability. 

Removing conflicts may increase delays

Trade-offs between comfort and convenience are inherent to decisions about separating conflicting bike and motor vehicle movements. 

Motor vehicle turning movements consume a large amount of time and space at intersections. At the same time, people on bikes express a comfort preference for protected bike signal phases with separate motor vehicle turn phases. However, in many urban intersections, fully separated phases will result in longer wait times for everyone: people on bikes, people walking, and those traveling by bus or private automobile. This can lead to non-compliance.

To limit delays for people using the bikeway:

• Prioritize short cycle lengths, including using half-cycles.
• Minimize the turn phase while maximizing the bike phase, increasing the percentage of cycle length dedicated to bike movements.
• Use an actuated turn phase rather than running it on recall.
• Coordinate signals to operate at or near bike speed rather than the vehicular speed limit, allowing people on bikes to arrive at intersections just before or during the green phase.
• Install bike detection ahead of the intersection to call or extend the bike green for people riding in the bikeway.
• Overlap bike and pedestrian movements with complementary motor vehicle turn phases.
• Prohibit vehicular turns entirely.

Where delays are excessive, it is more effective to provide flexibility to people walking and biking, such as allowing them to proceed even after motor vehicles begin to turn across the bikeway. Geometric design elements, such as corner islands or wedges and lane line hardening, should be used where permissive vehicular turns across the bikeway are allowed.

Shorter cycle lengths

Shorter signal cycle lengths have operational advantages for bikeways and other urban streets. With more opportunities to cross the street and shorter wait times, signal compliance by all users tends to increase, reducing the risk of fatal and injurious crashes. 

Shorter cycle lengths also reduce exposure to turning vehicles. Though the same total number of drivers may turn, fewer turn during any given green phase. As a result, phase separation may not be necessary for safety.

While shorter cycle lengths result in lower vehicle throughput per cycle, they also result in shorter, more manageable motor vehicle queues. Shorter cycle lengths can improve motor vehicle operations when turns make up a high percentage of total vehicle movements. 

Prohibit left-turn movements 

Finding a safe time within a cycle for motor vehicles to turn can be challenging at complex or high-volume intersections. Left turns across bikeways at signalized intersections on two-way streets pose special safety concerns for people on bikes. Drivers turning left use most of their decision-making capacity to find a gap in oncoming vehicular traffic. People in the bikeway or crosswalk are the last consideration.

Analyze intersections along the corridor to determine if turns can be rerouted to other intersections within a reasonable distance. Alternatively, look for opportunities to convert a left turn into a series of right turns.

Rerouting left turns can also improve operations for transit vehicles and through vehicles.

Mitigate impacts to transit

Protected right-turn phases may be needed in locations where bus routes turn right due to sightline challenges between right-turning buses and through bikes. Phase separating these turns can affect transit operations at high-volume turn locations if the right turn queue is long. However, phase separation is also an opportunity to provide transit vehicles with a leading interval at the same time as parallel bikes. Bending the bikeway out toward the sidewalk to create a larger setback can minimize the time a person biking is outside the view of the bus operator and, in some contexts with low turn volumes, may be an alternative to phase separating the right turn. 

Creating a dedicated right-turn phase along transit routes may negatively affect turning capacity and through movements by transit vehicles. When a bus needs to serve a nearside stop, it may block right-turning vehicles during the entire right-turn phase. Alternatively, a bus may be delayed if it has to wait in a right-turn queue before reaching the stop. Relocating the bus stop to a farside or midblock location can help mitigate these operational challenges but may result in other undesired outcomes, such as longer walking distances for transfers or less crosswalk compliance.