Hope it's okay to reply to such an old thread but it's exactly what I've been looking to find!
Duane, did you ever end up testing multiple iR detectors with different ranges with multiple vehicles/matching emitters?
I was following your build back in 2012 and very much enjoyed the build-along (link to related thread here). It would seem you've moved onto other things since then, but if you have a minute to reply here, I'd be awfully appreciative! If you're reading this, please also consider sharing your most recent code for this excellent little project. Your blog says to ask you here on the forums, so here I am - please send if you can. Thanks very much indeed!
(I may very well try multiple detectors and emitters myself if I don't hear back - will post here if/when this is done so that this old thread may finally get some closure! Cheers).