well your right, cuz thats how it operates. if your the only car on the road and a radar detector goes off, your screwed.
it can read the future, it detects each time he shoots. so what your really doing is catching left over signal from cars infront of you. they get shot, your detector beeps, you slow down, he shoots you, it goes apeshit. but your slower
I wouldn't call it a waste of money if you're constantly speeding, neither is it a bible of assurance to being immune to getting a traffic ticket. It's simply another device/gadget designed to provide warning if any of any radar threats.
Radar detectors are great, but like Noel said there is no assurance that you wont get a ticket. What I do is, I always try not to be that one lone car speeding down the highway and I like to keep about a 1/4 mile between me and the lead car.
Your speed can also be gauged in several other ways theres VASCAR, LIDAR and old fashioned pacing. There's even a new radar gun that is being sold that doesn't even work on the same scope as radar detectors- I wish I could tell you more on this one but I threw away the ad for this item a while ago.
Take a look here do your research and remember, you get what you pay for.
what you've already heard is true...catch 'em catching the next guy. you want a sensitive detector that can also tell the difference between trouble and junk, but there are few (none?) that do both well. you will also learn to distinguish patterns to aid identification.
i've never owned a car without a detector (an original escort), and in fact wired one into the cowl of a suzuki i used to own. i currently have a one-generation old valentine one, and i like it a lot. it is very sensitive, but has only adequate filtration. it also has two *great* features that few others do:
a counter indicating individual sources. here in NC there are radar emitters in bridges and construction signage. with a normal detector, you learn the bridge is always noisy, you speed through, and you don't realize that today there are two sources of radar...the bridge and johnny.
a directional indicator. faint signal, getting stronger. where is it? it's nice to know ahead/behind if it's legit, and sideways helps identify false sources.
worth the premium? apparently i think so. has it ever saved me? you bet. i believe the only state in which they are still illegal is VA / DC. canada too, i believe. and perhapts CT.