Thursday, March 08, 2018

Wronged by Radar

Radar merely displays a speed, leaving it up to the officer to figure out which vehicle it's looking at. Not infrequently, the officer gets it wrong.
In Florida an elderly man was outraged when he was pulled over by a Florida Highway Patrol trooper and ticketed for driving 77 mph in a 65 mph zone. On I-80 in Wyoming a driver was cited by a trooper for 91 mph in a 75 mph zone. A Porsche driver was stopped by a California Highway Patrol trooper and ticketed for driving 15 mph over the limit.

All three were innocent: the officer was wrong in every case. This is hardly unusual and plenty of innocent drivers get speeding tickets from radar-equipped cops.

After speaking with the drivers and analyzing the evidence, in each case it was apparent that the officer either was clueless about radar operation, failed to follow procedure or wrongly identified the offending vehicle.

Mistakes are made because traffic radar merely displays a number, leaving it up to the officer to determine which target is producing the speed.

When used in Moving Mode, the radar also shows the cruiser's speed, called Patrol Speed. And in the Fastest Speed mode used by these officers, a third speed is displayed, that of the fastest target within range. This is a lot of real-time information for the officer to be monitoring.

Moving-mode radar can get confused, especially in traffic. For this reason, officers are trained to carry out a multi-step process to verify that the radar is working correctly and they've identified which speed is being generated by which vehicle, something called tracking history.

To establish tracking history the officer must:
  • Estimate the target's speed

  • Compare radar patrol speed—the cruiser's—to the speedometer and verify that the two match

  • Observe radar target speed

  • Verify that target speed approximates the visual estimate

  • Verify that the radar's Audio Doppler conforms to the visual speed estimate

  • Observe target vehicle and target speed for a few seconds, making sure they remain relatively constant
All three troopers were using Stalker DSR radar, an excellent unit with phenomenal range. One of its unique features is full-time Fastest Speed mode. This allows it to continuously display the speed of the strongest target and also the fastest target.

Stalker DSR 2X sibling of the DSR can monitor twice as many targets simultaneously. Green 51 mph is Patrol Speed; 36 mph speed (upper left) is the approaching minivan at left. The 54 mph Fastest speed is from the car behind the minivan. 

The strongest target, the one returning the strongest reflected signal, is usually the closest vehicle.  But not always. For example, the radar might clock an 18-wheeler 800 feet away while ignoring a bite-sized Ford Focus that's much closer. It will begin reading the Ford's speed only when it has closed to nearly point-blank range.

In the Florida case, I concluded that the 77 mph target was a big rig far behind the Cadillac driven by the retiree. My guess is that the trooper hit the Transmit button, saw a 77 mph Fastest speed appear and assumed it was the closest vehicle, the Caddy, which she pulled over.

She may have been unaware that her radar was capable of clocking an 18-wheeler a mile or more away. Regardless, if she'd have taken her time and followed procedure, the wrong guy wouldn't have been ticketed.

In analyzing the rear-camera footage from the Wyoming trooper's in-car video system, the Dodge clearly wasn't gaining on the cruiser; if anything, it was falling back.

The driver confirmed that he'd spotted the marked Wyoming Highway Patrol Ford and lifted from the gas, the typical reaction. He said the trooper had abruptly accelerated, passing a tractor-trailer in the right lane, and continued for about a quarter-mile. Then the Ford pulled onto the shoulder and when he passed by, fell in behind and lit him up.

In the video, no vehicle other than the defendant's slowing Dodge could be seen for at least half a mile behind the cruiser.

That left two potential sources for the 91 mph on the ticket. Although none was visible in the video, it's conceivable the radar was looking at an 18-wheeler nearly back over the horizon. On the eastern prairies of Wyoming, 91-mph big rigs aren't uncommon.  

Wyoming Highway Patrol cruiser's rear video camera captures the likely source of a 91 mph speed wrongly attributed to an innocent Dodge driver.

But the video offered a more likely explanation: The 91 mph violation speed was probably generated by the spinning wheels of the 18-wheeler the trooper was passing at the moment. If he'd been more accurate in estimating the speed of the Dodge, he'd have noticed that he and the radar were looking at different vehicles. But it's tough to estimate speeds by squinting into a rear-view mirror.

And in the California case, the rookie trooper testified that the distant Porsche he cited had definitely produced the 80 mph Fastest speed, not the car that was barely 200 feet away from his CHP cruiser. He was wrong, but the judge still found the guy guilty.