[RichD]

Are you using a common ground for all of the lanes? That is known to cause problems. It is safer to run seperate grounds for each lane. Poor emitter to sensor alignment could be a problem, especially if the sensors are recessed a bit. Dust can get into a recess and block the light. If nothing else works you might switch to a serial port setup.

 

[RichD]

You need to find out if this is a hardware or a software problem. A count either gets triggered by an open or closed circuit no matter what kind of sensors you use. If you disconnect the sensors and just touch the wires together you can duplicate the situation when two cars cross at once. If you do two sets of wires at once and the count is OK then your problem must be with the sensors. If you get the miscount the problem is with the cable, the computer port or the software. You never did say if you checked for a common ground wire, that is a known issue with systems of this sort.