I would try a narrow beam US sensor pair, this model would probably be best... your accuracy will still be off though...
http://www.maxbotix.com/MB1240__XL-MaxSonar-EZ4.html
You should look around their various products (listed on left side), each has its own range in terms of distance and width of measuring "cone"... You would want one that measures far enough to reach the side of a car, but with a width small enough to be accurate when determining if a car has come in front of it and when the car has moved past the sensor...