Being Predictable
- for Drivers

Bicycling Safety for Drivers  
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3 Being Predictable


Summary: The parallel track for cyclists encourage cyclists to "be predictable": to follow normal traffic laws. The following provides an overview of how motorists can expect cyclist (at least, experts) to behave. It is worth remembering, though, that novices, especially children, can be unpredictable, and additional care is warranted.

3.1. Vehicular Cycling

For a bicycle speed of 15 mph, at 0.57g you can stop in 13.1 feet and your turning radius is 26.2 feet (corresponding to leaning the bicycle 30 degrees from vertical). Such stopping distances and turning radii are typical of cyclists with a high skill level, and both values can be even larger for less skilled cyclists. In any event, both of these values are large enough that a cyclist has to behave like the driver of a slow vehicle rather than like a pedestrian. Expert cyclists will typically

Less experienced cyclists, and particularly children, may behave erratically.

3.2 Position on the Roadway

Cyclists will position themselves on the roadway so as to minimize the risk of an accident. Accidents in which a cyclist is hit from behind occur infrequently, and most collisions (and for that matter, most fatalities) involve conditions that occur ahead of the cyclist. It is particularly dangerous to be constrained with no room for maneuvering. An appropriate position on the roadway is shown in the next figure. The figure shows several lane widths, but the width of each section and the distances between vehicles have been shortened.

Illustration of optimal cyclist position on the roadway

In this figure,

Bike 2 would be safer by following the path used by Bike 3, even though this would put Bike 2 in the path of traffic (merging into traffic, of course, has to be done carefully). Expert cyclists will typically follow the path shown for Bike 3 if the lane is too narrow to be safely shared. Except for some rural roads that typically have little traffic, the most frequent case in which cyclists have to use a full lane occur near intersections, where additional turn lanes have been squeezed in to improve the level of service at the intersection. Such narrow sections are typically short.

3.3 Changing Lanes

In order to avoid miscommunication of intentions with drivers, experienced cyclists will change lanes as follows, and drivers should be aware of this practice:

  1. The cyclist should first look over a shoulder to see if there are overtaking vehicles.

    Initial phase in preparing for a lane change

    In some conditions (particularly near reflector dots or when it is necessary to brake), a cyclist may not be able to give a normal hand signal. If a cyclist looks alternately over his/her shoulder or straight ahead, drivers should interpret that as a signal. The cyclist will probably take some concrete indication from the driver, such as slowing to the cyclist's speed, as an indication that the driver will "let" the cyclist into the lane.
  2. Once the cyclist has determined that a lane change is safe, the cyclist can cross the lane stripe. The cyclist may check again by looking over a shoulder before preceeding across the lane.

    Second phase of a lane change
Note that it takes a few seconds for each phase, and you should allow at least 5 seconds to get the attention of an overtaking driver. At 15 mph (22 feet per second), a cyclist will have to leave couple of hundred feet as the bare minimum distance needed to cross a traffic lane.

3.4 Intersections

Expert cyclists will typically choose a path through an intersection (as allowed by traffic laws) so as to minimize the number of lane changes. In the typical intersection, this means that

For turn lanes that are left-turn or right-turn only, the positions are illustrated in the following diagram.

Cyclist position in intersections with no shared through and turn lanes

For the case where there is a lane that can be used by turning and "through" traffic, the positions are illustrated in the next diagram.

Cyclist position in intersections with shared through and turn lanes

A cyclist will ideally be in a position in the lane so that drivers will not cut the cyclist off by turning across the cyclist's path. When stopped (at a stop sign or red light), expert cyclists will often wait in the center of the lane. Then if a car that is about to turn shows up, you can move the bicycle to the side to let the car by. Cyclists who do this will look over their shoulders to check the turn signal, so it is important to use the turn signals even though the cyclist is ahead. Once the cyclist can proceed, the cyclist will move to the normal position on the roadway.

3.5 Riding in Traffic

While, as stated above, a cyclist may use a full traffic lane when the lane is too narrow to share with a motor vehicle, a cyclist may also ride in the stream of traffic when going as fast as motor vehicles. The following figure shows why This is necessary.

Cases in which a cyclist should ride in traffic

Note how the cyclist is in a position where the vehicle ahead cannot cut the cyclist off by turning. Drivers should expect to see cyclists riding in this way.

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