Filtering forward

Filtering forward, or filtering, is a technique used by bicyclists and motorcyclists to pass a stopped or slow-moving lane of congested traffic by traveling in unused lane space. When the space used is between two lines of vehicles, this is also known as lane splitting, but filtering can be accomplished by using space on the outside edge of same-direction traffic as well. There can be a significant saving of time by bypassing what otherwise would be obstructions.

Safety

To safely filter forward, in stopped or extremely slow traffic, one should travel extremely slowly, being aware that vehicle doors may unexpectedly open. Also, unexpected vehicle movements such as lane changes and such are possible without much warning. Extreme care needs to be taken around buses and tractor trailers, as the cyclist may be nearly invisible to the drivers (who typically aren't expecting a bicycle filtering forward). To avoid a right hook collision at an intersection, if one filters forward to the intersection, either (1) position oneself to the "front" and "right" of the stopped lead vehicle, or (2) stay behind the lead vehicle. Do not stop directly to the right of the lead vehicle, as one is less visible there.

Etiquette

Generally speaking, it is poor form to "unnecessarily" force other vehicles to pass oneself more than once in a short time span. As such, if one was just recently passed, it is considered poor form to filter forward to force the vehicles to pass again when traffic resumes moving at speed. Filtering forward is never required. If one is holding one's position in stopped traffic, one should take the lane to avoid being squeezed.