Cars pass through ProxyVehicle

[mquinlan]

1) move a PV to the middle of a lane
2) cars just pass through PV as if PV is not there
This is probably b/c the cars check for things in front of it by checking only vehicles in its lane (or something like this), so it doesn't even check if the PV is there. This behavior understandable for computational reasons but makes it harder for us when our real car changes lanes or swerves off the normal route.