Radar with Same Direction can clock targets ahead of or behind the rolling cruiser. Not only is the Ford pickup truck ahead an easy target, but the distant Harley-Davidson's speed is a button-press away for this Decatur Genesis II Select radar. It doesn't matter that the 18-wheeler is a far stronger target.
Target speed of 64 mph is the white truck; 61 mph Patrol speed displayed is the cruiser's. Amber LED indicates that the front antenna has been selected and is transmitting.
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 story.
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.
The best rear detection range we've witnessed by a late-model detector was by the Escort Redline. It reliably detected X, K and both popular frequencies of Ka-band radar when they were still 1.25 miles behind us.
During another test, this at our unlimited-range site—the one where another Escort Redline set the world-record 14 miles of radar detection range—the test Redline continued alerting to the tripod-mounted Ka-band radar behind us for 4.1 miles as we drove to the opposite end of the test site.
Why the big increase in range? We were following an 18-wheeler whose rear trailer doors were covered with polished aluminum, turning it into a giant microwave reflector. Small wonder we rank the Redline as the best-performing windshield-mount model on the planet.
We were curious to see if the claim by one manufacturer in particular, whose Valentine One has a second, rearward-facing antenna, makes it the only real defense against radar coming from behind. We comparison-tested the Valentine One and Escort Redline and as expected, the V1 had longer range against radar coming from behind.
The V1's near-equal radar range, front and rear, does have a downside, however. In a 750-mile road test of the BEL Pro GX65 (now called the BEL Pro 500) to measure this GPS-enabled model's false-alarm resistance, we used a Valentine One for comparison. On that trip, the V1 consistently alerted to the X-band relay radars used on cellphone towers--for up to 15 miles when approaching one, and another 15 miles departing. With no automatic audio muting, each interminable event was punctuated by constant stabs at the mute button to silence the din.
The number of Valentine One alerts recorded on the trip was 133, only six of which were police radar. The BEL spotted the same six radars while uttering two false alarms on the outbound leg and none on the return. This is the reason why the best GPS-enabled radar detectors enjoy huge popularity today.
To determine the real-world threat level of radar coming from behind, we set up a second test at our usual site in the desert north of Phoenix. By coincidence, we'd spent the previous month field-testing every frontline moving radar model on the market, the mainstay of the nation's state highway patrols, 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
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 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.
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.)
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
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.
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
How Same Direction Radar Works
Here's an example. We're on a six-lane urban freeway, 55 mph limit.
The target is the black Ferrari 348 GTS, 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.