The Shared Lane Marking (click for 14 page paper) (SLM) is for use in lanes too narrow to share side-by-side. They lane must be “shared” sequentially, one-after-another, as how motorists “share” a lane. According to the MUTCD it is interchangeable with the R4-11 BICYCLES MAY USE FULL LANE sign, which informs the correct use of the marking. A better name would be the Bicycles May Use Full Lane Marking.
The SLM has minimum placement specifications that are flagrantly faulty. When placed at minimum specifications, the Shared Lane Marking encourages side-by-side use, acting like a faux Bike Lane on roads that are too narrow to have a Bike Lane, or else leads bicycle users into the dangerous Door Zone of parked motor vehicles.
Used at minimum lateral specifications, the SLM is at best “security theater” that attempts to micro-manage bicyclist lateral position, making some bicyclists feel safer while luring them into hazardous positions. Such fraudulent use to fleece the traveling public is a foul flouting of engineering responsibility.
The Shared Lane Marking should be placed to covey the same message as it’s companion R4-11 sign. The SLM should be placed neutrally in the center of the usable lane space, as would directional arrows. The SLM could be accompanied by the R4-11 sign and optional CHANGE LANES TO PASS supplemental plaque.