Radar from Behind: The Real Story

Judging from all the Internet chatter, it would appear that police have unleashed a new doomsday weapon: a super-radar that can sneak up from behind and blast you without warning. The only defense, say these breathless missives, is a radar detector that incorporates a rearward-facing antenna. Like any good rumor this one contains just enough truth to lend it some credibility. But there's a lot more to the

Truth is, any sensitive radar detector can pick up radar coming from behind. The signal reflects from objects ahead of the detector and back into its antenna. Admittedly, some are far better than others, but they all have this capability.

But we were curious to see if the claims by one manufacturer in particular, whose hyper-expensive model has a second, rearward-facing antenna, made theirs the only real defense against radar coming from behind. So we decided to set up a test at our usual site in the desert north of Phoenix.

By coincidence, we'd just spent the previous month field-testing every automatic direction-sensing moving radar unit at this same location. We'd been retained by one of the major radar gun manufacturers to run a comparison test of their new unit along with all four competing models. These were the latest high performance, dual-antenna moving radars and all had Same Direction (also known as Same Lane) capability. This is the feature that enables the radar to clock vehicles moving in the same direction as the rolling cruiser. In the case of these twin-antenna radars, the officer can use the rear antenna to clock vehicles coming up from behind or the front antenna to clock vehicles ahead.

Same Direction Mode

We spent days testing this function and found that while indeed it works, it's severely limited in application. In the total absence of other traffic, the average maximum Same Direction range was barely 2,200 feet when we targeted the rear of our mid-sized 1999 Nissan Maxima target car. And no trained, conscientious officer would even think of clocking a vehicle at that distance. That's because by law, before using his radar he must first observe the speeding violation--a near impossibility at nearly half a mile. True, if the officer is doing 65 and the other car is clearly pulling away from him, he'll know the target is traveling faster. But at that range the speed differential could be 10, 20 or even 30 mph for all he knows. To find out, he'll begin accelerating, trying to match speeds and gradually close the gap.

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He does this both to get a better fix on its speed and to identify the vehicle make, model, color and driver ID, information required by radar case law, common sense or both. But isn't he concerned about being spotted by the speeder? Oh, please. The average driver doesn't check his mirrors from one month to the next and the faster he's driving, the more prone to tunnel vision he becomes. We've driven fully marked patrol cars one car length behind speeders at 90 mph for several miles without ever being spotted; they were so focused on the road ahead it never occurred to them to glance in the mirror.

Okay, but can't the officer accelerate to high speed and, as he's closing the gap, flip on his radar and clock the target? Not if he exceeds the radar's maximum allowable patrol speed. That usually works out to about 75 to 85 mph. Once over that speed his radar stops working and won't resume operation until he's slowed down again below that threshold speed.

In Same Lane mode most radars will clock the strongest target, allowing a smaller, faster target to escape detection this way. (But not if the radar has Same Lane/Fastest capability.)

There are additional limitations in the use of Same Direction radar. To make certain his radar is reading the intended target, the officer also must verify that no vehicle within range is traveling faster than the target. On moderate to heavily traveled multi-lane roads this is frequently impossible.

How Same Direction Radar Works

Here's an example. We're on a six-lane urban freeway, 55 mph limit. The target is a black Ferrari, running 90 mph about 500 feet ahead of the patrol car in Lane 1 (next to the median). He's about to pass an 18-wheeler running at 64 mph in lane two (right lane). The officer releases the RF Hold button, engages Front Antenna and bada bing! a speed appears in the Target window, 64 mph.

Although it's a bit farther away, the barn door-size target offered by the rear of the truck generates a much stronger return signal than the car. If the Ferrari pilot is packing a halfway decent detector, it's leaping off the dash and the game of cat and mouse is over.

Most radars have no ability to ignore the slow truck while in Same Lane mode, although they can do so when in opposite-lane/fastest mode. This means the fast-moving Ferrari gets a pass. Now you know why radar from behind can be a threat, but it's nothing to lose much sleep over. For all but the very unlucky, any detector with good rear detection range--3,500 feet and up--will see it coming in plenty of time.

Whoa, hold the phone, Bubba. You already knew all this stuff? Okay, then let's test your radar knowledge and find out how smart you really are.

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