I had a very similar incident some years ago. A police car overtook a firetruck in front of me. The 4x4 in front of me drove onto the hard shoulder (Which was grass) and I followed her in my little sports car and promptly slid all over the shop and bumped her up the bottom. I ahd no chance of stopping, but of course, as I hit her, it was my fault. Not very fair in the circumstances, but I have to take it on the chin. When I spoke to the fire brigade about it, their answer was....'what police car?'....grr!