RCMoeur 

From what I've seen, agencies are using shared lane markings in locations where there is notable bicycle demand or ridership, but current budgets and built environment don't allow for a different accommodation. For example, a built-up narrow street may be impossible to widen without destroying the neighborhood, but also serves a number of cyclist destinations and has lower travel speeds. Or it could be used where a roadway is intentionally kept narrow to encourage lower travel speeds.

A colleague of mine (John Ciccarelli) and I wrote an extensive discussion on how best to utilize shared lane markings for the second edition of the Traffic Control Devices Handbook published by the Institute of Transportation Engineers. However, this is covered by copyright and cannot be republished in full here.