I've heard every reason under the sun for these perspex thingies - everything from it being a big magnifying glass so they can read license plaates further away (!!) to a wind deflector to protect the light bar on the roof as they only stick it on with glue (!!!). LMAO You don't generally see these on the smaller urban police vehicles so I'm going with the debris-deflector idea. If the bigger faster cars that can be asked to undertake pursuit work need 'em then I'm guessing it's to protect the windscreen from flies but also stones etc. Getting a stonechip at 50mph doesn't compare to getting one at 140+ on the Wangan!!!