I think it's another one of those grey areas. If I was in that situation, I'd probably get out of the way provided it was safe.

DH got caught like that once on a freeway where the two left lanes were bumper to bumper (two lanes turning off) and the right lane was free. A police car came up behind him, lights and sirens going and DH floored it until he found a break in the traffic where he could slip in. The police car pulled up beside him, tooted and waved and then continued on their way. But I reckon he took a pretty big risk in doing it, but the police in the car were obviously grateful.