Pennsylvania’s “Four Feet Rule” requires motorists to leave at least four feet of space between themselves and bicyclists when passing. All of the responsibility for measuring this distance is in the hands of the car driver.
It is legal for drivers to cross over a double yellow line to get past a bicyclist provided there’s four feet of space. Of course, the driver should be on a straight road where s/he can see clearly in front of them for a few miles.
The “Four Foot Rule” also applies to how car drivers open their car doors when parked near a bike lane. To avoid slamming a door in front of a bicyclist, Pennsylvania requires drivers to leave four feet of space between their door and bike-protected lanes.
According to Philadelphia’s Bicycle Coalition, the best opportunities for cars to overpass bicyclists with four feet of clearance are usually at stop signs or red lights.
For more information on the “Four Foot Rule,” feel free to look through this webpage put together by PennDOT.