Ideas for a lap counter for RC cars?

Hi:

I am trying to design a lap counter for radio controlled cars. So
every time one of a dozen cars or so goes over the start/finish line
another "lap" is counted for that car. I plan to use a Windows based
PC (an old 166MHZ) to track the laps and times, etc. I think I can
handle the computer programming end of things (as a VB developer) but I
am not an electronics expert. The biggest puzzle I have right now is
figuring out how to transmit the cars ID (if you will) only 3-12 inches
to a receiver near the start/finish line. I am hoping for some help
from you experts pointing me in the best direction.

Here are some important points I had concerning the project so far:
1. Can count laps seperately for up to 12 cars. So it has to know
which car just crossed the line, etc.
2. It has to be reliable. No missed laps etc. So I figured dragging a
light "brush" on the bottom of the car is out. It may not make good
contact etc.
3. I considered RF and/or IR transmitter/receiver type of design. RF
seems nice because bright lights and/or line of sight isn't important
like IR. IR may have some issues if several cars are sending pulses to
the receiver at once. Example: a 40HZ cycle from car 1 and a 20 HZ
cycle from car 2 could look like a 60HZ cycle right?
4. Since RF is usually trying to transmit distances greater than the
3-12 inches required for this project, are there some other types of
circuit that I may be missing that would transmit this very short
distance (circuits that one may not usually think of as an RF
transmitter...but will transmit this short distance...if you know what
I mean)

If you can share any thoughts, ideas, advice or suggestions that would
help point me in the right direction I would greatly appreciate it.

Thanks!

Joe G.

.

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