Are the police hot on your trail and have set up a roadblock in front of you with spikes
well most of the time they don't put spikes all across the road allowing easy access and this may even stop some of the police cars

since they dont put them up all the way go for the opening whic is sometimes blocked by a car/suv so hit the end of the car/suv or the trunk and you SHOULD be able to keep going while some of the plice cars will hit their own spike strips causing them to drop out