Well, somethings you just cant get by.
"instant on", as you call it is VERY VERY expencive. If you think about it, for a vehicle traveling one way, to have radar "pointing"( and i use the term pointing VERY loosely) one direction, and traveling one direction, while clocking an oncomming target...There are a lot of variables there.
The radar system is so expencive because it has to be tied into the vehicles speedo, ECU and a bunch of other things as well, to actually be able to calculate the speed in which a vehicle is comming at you, while you are moving towards it.
Most Police depts use a passive ( pulse) radar..meaning that it emits a radar signal while they are sitting still ( and cant calculate when your moving). Its up to the officers discresion exactly what vehicle in the pack was traveling the fastest. The display in the cruiser flashes number after number, and again its the officers call who was traveling at how fast. Not accurate at all, unless you are alone, what they call the sitting duck. These units are cheap, disposable and you can get a unit in ever car for under 300 bucks.
Laser, as we know has to be used while sitting still...same idea here, but 100% accurate. There is 2 foot diameter the laser can clock...so when clocking you get the "that exact vehilce was traveleing this fast". There is no debating, unless of course the law decides to fib.
Most larger dept.'s, including Boston have dedicated traffic units. All day long, its moving violations and parking tix. These vehicles tend to be more equipt than a patrol car. Infact here in Boston 99% of our cruisers dont have radar. No time, no money for radar. Often the mobile units ( motorcycle) will radar, but need to use a laser just in essence of size. It can easily be put in the saddle bags and carried around. Watch out for dedicated traffic units though. They will have multiple radar units, working different ways an dcan pretty ,mcuh clock you how ever they want to.
The state police in some states have more money than they know what to do with..watch out. Often times they can get you however they want, from whatever direction, moving or stopped.
Just a Lil info for you guys..most likely, if a small town cop cites you for speeding while he was comming your direction, you can put a up a decent fight in court. Ask what type of radar he was using, and if it fits any of the above criteria. You may just find out, he really just "thought" you were going fast, when in reality, he had no way to truly know.
it pays ( well, in this case you wont have to) to be well versed in radar. Save your ass in court, and save you those points on your insurance and licence.
just fyi..Josh
some good sites related
http://www.beattraffictickets.com/?OVRAW=speed traps&OVKEY=speed trap&OVMTC=standard
for your next road trip..if your using a GPS, you can set these locations as icons, or even way points on your software, so you know when your approcaching
http://www.speedtrap.org/