When are bicyclists allowed to ride two abreast in a traffic lane?

Bicyclists are allowed to ride two abreast in a traffic lane specifically when the lane is of substandard length. This provision is in place to ensure safety and better visibility on the road. A lane of substandard length typically means a lane that is not wide enough to allow a vehicle to safely pass the bicyclists without leaving the lane of traffic.

In such cases, riding two abreast can help to establish the space the bicyclists occupy and can reduce the risk of dangerous overtaking maneuvers by other vehicles. It is a way to enhance visibility and encourage drivers to treat the bicyclists as a single unit, thus ensuring that there is enough space when they attempt to pass.

The other scenarios described in the choices do not provide the same safety measures or legal backing for riding two abreast, as they either imply conditions that are not conducive to safe cycling practices or misinterpret appropriate riding regulations. For example, riding on sidewalks usually falls under different traffic laws and might not even be permitted in many areas.