Building Safer Intersections - Cambridge Bicycle Safety

Introduction

Thanks to the tireless work of city councilors, staff, and advocates, Cambridge has drastically increased its bicycle mode share for all trips within the city. At the same time, cycling injuries and deaths have decreased significantly.¹

However, June 2024 has seen the tragic deaths of two people riding bicycles in the city. Both of these tragedies occurred at intersections and involved a turning truck. While the evidence clearly points to the fact that we have made great improvements to cycling safety in Cambridge, these incidents underscore the fact that there is more we can be doing, particularly at intersections.

Current Strategies

As the landmark Cycling Safety Ordinance, the law which requires the City of Cambridge to install separated bicycle facilities on all key priority corridors by 2026 or 2027 (depending on the street), is implemented, city staff have worked with stakeholders to design, refine, and construct “quick build”, partial construction, and full construction projects across the city. These types of installations are differentiated by the type of construction materials used, whether it is paint and plastic “flex post” delineators in the case of “quick build” projects, to granite curb and asphalt cycle tracks in the case of full construction projects, or a mix of both.

Quick-build separated lane on Hampshire Street

Full construction cycle track in Inman Square

Both of these implementation techniques have improved the safety and comfort for people biking by separating them from motor traffic and eliminating conflicts with parking vehicles.

However, when separated bike lanes meet most intersections in Cambridge, they often lose most of their physical protection. City engineers have mitigated potential risk for vulnerable road users (people walking, cycling, or using other non-car mobility devices) in several different ways.

Daylighting

Parking is removed from areas close to intersections and crosswalks in order to make vulnerable road users more visible to people driving.

Turn hardening

Corners of intersection are extended to require turning vehicles to reduce their turning radius and as a result take turns more slowly. This has been implemented with both paint and flex posts and curb and concrete islands.

Signal Phasing

Conflicts between vulnerable road users and motor vehicles can be reduced or eliminated through signal phasing. For example, signal phasing can separate bike and vehicle movements in different signal phases to eliminate conflicts like right hooks, where a right turning vehicle strikes a person in the bike lane continuing straight.

However, like all safety treatments, it is important to note that separate phases alone are not sufficient and have disadvantages. Layers of protection are needed so that if one fails, danger can still be mitigated. Because this approach relies on all users recognizing and following signals, it is critical that they are deployed in a way that is visible and legible. Bicycle signals in particular need to be placed in the field of view of a person on a bike, which is often different from the field of view of someone driving a car. Also, in order to have separate signal phases for motor vehicles turning versus going straight, the intersection is usually required to have additional lanes for motor vehicles, such as turn lanes. This requires allocating more cross-section to moving motor vehicles. In a space-constrained setting, this can lead to trade-offs between signal separation and physical separation between motor vehicles and vulnerable road users.

A diagram of a separate bicycle phase that eliminates conflicts with right turning vehicles²

A 4 inch, near side bike signal. Human scale signals are much better for controlling
bicycle movements, and are the standard in many European countries.

Another technique that is used when creating separate phases is not feasible, is a Leading Bicycle Interval, or LBI, which gives people biking a 2-6 second head start before the light for vehicles turns green. This allows vulnerable road users to become visible to turning vehicles or even cross entirely through the intersection before other vehicles start moving.

A diagram illustrating a leading bicycle interval as well as a lagging left turn,
which also helps prevent conflicts between pedestrians, cyclists, and left turning cars.³

Weaknesses and Implementation Failures

While these strategies are shown to help reduce crashes, on their own they are not enough to achieve the only acceptable number of serious injuries and deaths on the road: zero. According to the Safe Systems approach developed by the US Department of Transportation, humans can be expected to make mistakes and therefore redundancy is needed so that one failure does not cascade into a tragic outcome.⁴

A road user may not see or may ignore a red signal or prohibited turn sign. A driver may choose to park their car illegally in a bike lane or daylighting zone. While effort should be made to reduce these occurrences, the design of our streets should prevent one failure or poor decision from becoming fatal. Cambridge should prioritize using a combination of these strategies, along with additional ones described in the following section, in order to add these critical layers of redundant safety.

Details of the implementation of these strategies may also lessen their effectiveness. Bicycle signals must be visible and obvious to people traveling through the intersection for the first time. This problem is compounded when the design of intersections varies widely across the city, rather than a limited set of standardized layouts being used.

Daylighting and turn hardening materials must be durable and do their job of preventing vehicles from occupying the space. Flex posts don’t do a good job of this and may become damaged or destroyed after being driven over repeatedly. Many quick-build materials need constant monitoring and repair or they will lose their effectiveness.

Additional Intersection Treatments the City Could Use

Protected Intersections

A protected intersection design is considered by many to be the gold standard for protecting vulnerable road users. It works by extending the physical protection of the bike lane as far into the intersection as possible, while incorporating other elements like signal separation, a reduced turning radius and better lateral separation between the bike lane and motor traffic.

Protected intersections utilize wedge-shaped corner islands which function to both separate bicycle traffic from motor traffic for as much of the intersection as possible and also modify right turns so that drivers are facing the bike lane by the time they must cross it. This encourages slower turning, and also provides better vision of and ability to yield to oncoming bicycle traffic.

Diagram of a protected intersection during a “scramble” phase, where people walking and on bikes are able to have full use of the intersection while other traffic is stopped. Other signaling strategies may also be used depending on the context.⁵

Protected intersections are ideally fully constructed with curbs and concrete, but this investment does not need to be made up front in order to receive safety benefits. They can also be created with “quick build” materials similar to other installations the city has done in recent years.

A protected intersection in the Netherlands

⁶A protected intersection in San Jose, CA built with paint and flex-posts

Harder Turn Hardening

As discussed earlier in this document, many street projects done in Cambridge incorporate some amount of turn harding, which is intended to reduce corner radii and slow turning vehicles. However this approach, as implemented, has several shortcomings.

Quick build materials like paint and flex posts are limited in the amount of protection they can provide. They do not physically prevent vehicles from crossing over them and taking turns at higher speeds. Additionally, city engineers are limited in how much they can reduce the turn radius due to the presence of larger vehicles like trucks and buses that need to make the turn.

There are different solutions that can accomplish the goal of slowing vehicle speeds on turns while also being durable and accommodating trucks.