Archive for August 19, 2012

Bike Boxes

The National Association of City Transportation Officials defines “bike boxes” as follows:

A bike box is a designated area at the head of a traffic lane at a signalized intersection that provides bicyclists with a safe and visible way to get ahead of queuing traffic during the red signal phase.

A typical installation is shown below. Bicyclists pass queued motor traffic on the right and can then move left into the bike box to wait ahead of motor vehicles before going straight or turning left.

Typical bike box installation in Oregon

Bike boxes are not approved traffic control markings in the Manual on Uniform Traffic Control Devices (MUTCD) and are not included in the American Association of State Highway and Transportation Official (AASHTO) Guide to the Development of Bicycle Facilities.

Bike boxes suffer from two fundamental design flaws:

  1. They direct bicyclists to pass on the right side of traffic that may turn right at any moment.
  2. They direct bicyclists to turn left and cross in front of other traffic with no signal-protected clearance interval at near zero sight distance.

Right Hook Hazard

A marked curbside bike lane directs straight-traveling bicyclists to pass to the right side of motor traffic that may turn right, in violation of the normal destination-positioning traffic rule. A motor vehicle driver in the general purpose lane can turn right with no advance warning to the bicyclist during a red signal or when the signal turns green. If the motor vehicle is a truck or bus, the rear wheels will track to the inside of the turn and crush the bicyclist. This is an increasingly common cause of fatalities to bicyclists in urban areas.  The right-turning driver’s attention is focused on traffic coming from the left and on traffic ahead of his path, and not on unexpected traffic overtaking in his blind spot.

Right Hook

Right hook hazard in curbside bike lane


There is no time when it is safe for a bicyclist to overtake to the right of a vehicle at the stop line.  To avoid this hazard, bicyclists and motor vehicle drivers who might turn right should merge in line with one another and sort themselves by destination before reaching the intersection. Many bicycling safety experts recommend ending bike lane markings well in advance of an intersection to facilitate this.  This is why California’s roadway design standard requires the bike lane stripe be either dropped or dashed within 100-200 feet of the intersection, and the California Vehicle Code requires motorists to merge to the curb – into the bike lane – when approaching a right turn.

Left Turn Signal Change Hazard

The bike box provides a waiting area for left-turning bicyclists ahead of queued motor traffic. By design, the bike box directs bicyclists to move into this waiting area from the bike lane “during the red signal phase.” If the signal turns green while the left-turning bicyclist attempts to move left into the waiting area, motor traffic may start moving with no warning to the cyclist, and motor vehicle operators responding to the green signal may not notice the bicyclist crossing suddenly in front of them.  This is especially hazardous in multiple-lane installations where traffic in one lane blocks view of the crossing bicyclist from the driver in the next lane.

Signal Change Hazard

Signal change hazard for bike box user

If the left-turning bicyclist is still in the bike lane when the signal turns green, the bicyclist must wait in the bike lane for a red signal or a large gap in traffic before they can move into the left turn position. By stopping in the bike lane during a green light, the left-turning bicyclist creates a potential conflict for other bicyclists who wish to travel straight or turn right.

Normal Execution of Left Turns

The safest way for a bicyclist to perform a left turn in urban traffic is to merge into the appropriate left turn position in advance while approaching the intersection like any other driver. This is done by looking for a traffic gap which, if not already present, can be created by signaling one’s intentions to other drivers. If the first driver does not provide space for merging, the following driver most likely will, or failing that, the next driver. The cyclist must begin the merge operation early enough to find an adequate gap or cooperative driver before reaching the intersection or stopped queue. This is a basic bicycle driving skill that can be easily mastered by anyone capable of learning to drive a car. However, on some roads with multiple through of lanes of high-volume high-speed traffic, some cyclists may feel uncomfortable or intimidated doing this, which may lead transportation planners to consider other possibilities.

Pedestrian-Style Left Turns

A pedestrian-style left turn is executed by the bicyclist by traveling straight across the intersection, stopping at the corner, turning 90 degrees in place, and continuing across the second leg of the intersection when the next green appears for that direction. The bicyclist may use the roadway or the crosswalk depending on the intersection design and local laws. The main disadvantages of this method are greater delay than a normal left turn, and potential conflicts with right turning traffic.

Pedestrian Left Turn

Pedestrian-style left turn

Cross-Street Bike Box

A cross-street bike box facilitates a variation on the pedestrian-style left turn where instead of traveling all the way to the opposite corner on the first leg, the bicyclist stops in a reserved space between the crosswalk and advanced stop line for the through lane traveling across the second leg. This keeps the cyclist out of the path of right-hook conflicts for the second leg of the crossing.  Because the bicyclist entering the cross-street bike box does not overtake on the right side of traffic and is provided clear signal protection with clearance interval, the bicyclist can enter the cross street bike box safely.

Cross-Street Bike Box

Cross street bike box for alternative pedestrian-style left turn

Separate Signal Phases for Bikeways

The only reasonably safe way to protect bicyclists turning left or traveling straight through an intersection from a position to the right of traffic with a conflicting destination is to provide separate signal phases for the bikeway and the general purpose lanes with right-turn-on-red prohibited. This has been done on many bikeways in Europe, and in some places in the US, such as the intersection of Broadway and Williams in Portland, Oregon.  The disadvantage of using separate signal phases is that this reduces the green time available for motorists and bicyclists and consequently increases delay for both user groups. The increased delay for bicyclists also results in lower rates of bicyclist compliance. For these reasons, separate signal phases for bicyclists are rarely used in the United States.

[UPDATE]: See this Portland Mercury news story about increased crash rates reported by the city of Portland at intersections where bike boxes were installed: City Finds Bike Boxes May Actually Increase Crashes