To get out of roadblocks easier, try looking for a fence you can drive through or just cut across some grass or the pavement. If you cannot manage any of these, build up some speed and bash into one of the police cars. Hit the front or the back, otherwise you will not get anywhere. Doing this will probably cause you to flip your car over - try to steer in the correct direction to help keep your balance. When you emerge from the other side of the roadblock, you should have only one police car after you.