Car counters

Hello. I have to make a simulated parking floor that reports how many free parking spots are available. It doesn't need to be very complicated (not necessarily individual car detection). I was thinking on something that detects the vehicles upon entry and counts them, substracting this number from the total. I don't know where to start so your help would be much appreciated.
Thanks.

you have 2 possibilities..... magnetic and optical sensors. magnetic sensors are like the buried coils of wire at a stop light, and optical would be like the devices that ring a doorbell when you walk into a store.

Ok. Both of them use a microcontroller?
Wich one is more simple?
Where can I find such circuit schemes?
Thanks.

Using the beam interruptor would be easiest to implement....


Food for thought...
I may be way off in my thinking and there may be a better way of doing this....
Starting with the max number of spaces available....say 1000, then subtract from that each time the beam is interrupted as a vehicle enters...but...you need an interruptor on the exit..so you add to the (1000-cars entering) remainder...