Felkebg,
The laser beam does disperse as it leaves the projector. However, the dispersion is actually fairly narrow, roughly the diameter of a mid-size car at 400 feet. Thus, you may get a signal from your detector if you were right next to the car being painted, or if some light reflected from a nearby car, which means that you are just as likely a target. Thus, no early detection is likely.
What this means is, the laser detector really only can detect when you are being painted. Does it work quick enough? My real life experience answer is "just barely." In maryland last year (not in the vette,) my laser detector screamed. I slammed on the brakes (which could be dangerous if you have tailgaters,) and the trooper promptly pulled me over. However, he said he clocked me at 70mph, when, right before braking, I was going 80mph. He decided not to ticket me, but he knew I was going faster. So, the detector kind of worked, but early warning is just not there.
sorry for the long winded response...
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-derek Z06 Black/Mod.Torch