A high Tech approach would be to use multiple IR sensors. The bushes may/will obscure them but if you take the average of the ranges, then the higher average should indicate the cars are in the drive way.
These can be mounted easily, but are expensive. IR Reflectance Sensor
A cheaper solution would be to maybe use laser beams and photoresistors, but with the same method of averaging the number of pins that go low. (High = not blocked, Low = blocked)