Can airplanes really check your speed when you're driving?

September 28, 2006 8:37 AM

You know those "Speed Checked By Aircraft" highway signs? I always use those as guide to knowing when there are no cops around whatsoever. But it got me to thinking: if there were in fact such aircraft COULD they check our car speed with radar? Is such a thing even possible?
posted by rileyray3000 to Travel & Transportation (28 answers total)

They check it not with radar but with large perpendicular markers on the highway and a stopwatch. It's not very accurate, but apparently it holds up fine in a lot of courts. Personally I've never seen an air patrol, so I rarely sweat it much... plus I think since they can see a bigger sweep of the traffic they'll spot the real speed demons and wait for them.
posted by zek at 8:40 AM on September 28, 2006

that's not how they check your speed from aircraft. the airplane measures the amount of time it takes you to move across a known distance. it typically holds up very well in court, too.
posted by mcstayinskool at 8:41 AM on September 28, 2006

If you notice those signs, look down at the road you're driving on. You will find that there are painted lines running perpendicular to your direction of travel. They use the time it takes you to travel between the lines to judge your speed while flying overhead.
posted by ninjew at 8:47 AM on September 28, 2006

they do that out in the mojave desert a lot. the cops fly a little cessna 172 I think. it's pretty simple to spot though ... see a low flying airplane? slow down. it's purely visual, so the cop needs to be flying pretty close.

having a convertible helps.
posted by krautland at 8:53 AM on September 28, 2006

and yeah, they use 'em. my first speeding ticket involved aircraft detection. now i watch out for them. look for lighter, small plaines moving parallel to the highway, and usually about 1-2k feet above.
posted by lester's sock puppet at 8:55 AM on September 28, 2006

You know those "Speed Checked By Aircraft" highway signs? I always use those as guide to knowing when there are no cops around whatsoever.

i suggest you question that logic ... if an aircraft does check your speed, who's going to pull you over and give you the ticket?
posted by pyramid termite at 8:55 AM on September 28, 2006

pyramid: they radio down to a cop working on the side of the road.
posted by lester's sock puppet at 8:57 AM on September 28, 2006

Interstate 10, between Jacksonville and Tallahassee, Florida, has these white lines crossing the pavement. I have also seen aircraft buzzing overhead in repeating patterns, so I assume it's used and it works.
posted by NationalKato at 9:02 AM on September 28, 2006

lester's sock puppet: I think that was pyramid's point.
posted by puffin at 9:02 AM on September 28, 2006

It certainly works, sometimes quite effectively.
posted by Skorgu at 9:11 AM on September 28, 2006

Answers to your questions: Yes. Yes. Yes. It's simple trig if the speed and angles are constant.
posted by blue_beetle at 9:12 AM on September 28, 2006

Oregon uses this extensively on I-5. You'll see a light plane flying overhead with 'STATE POLICE' painted on the wings. And yes, they use the equivalent of VASCAR. If there's any doubt or they really wanna peg you, they'll just route a statie to be watching for you from behind a bush.
posted by SpecialK at 9:13 AM on September 28, 2006

Obviously when the plane is out it is a coordinated effort so they do have cars on standby to get the speeders. It's not like one day they suddenly say, "Hey let's the take the plane out to check for speeders!"

You can pretty safely assume that they do not use this method at night or during foggy or stormy weather. It's a fair weather phenomena.
posted by JJ86 at 9:47 AM on September 28, 2006

Yep, these are definitely real. An alarmingly high percentage of people I know in Minnesota have been nabbed by WI highway patrol just after crossing the St. Croix into Wisconsin.
posted by j-dawg at 9:48 AM on September 28, 2006

Best answer: Note: When you do see the plane, don't just slow down; STOP. Even if you drop to the speed limit after you see the plane, your average speed (Way too fast + speed limit / 2 = slightly too fast) will still be over the limit and you'll still reach the second checkpoint under the time limit (bad).
posted by ChasFile at 9:58 AM on September 28, 2006

On the other hand, you can find these signs posted around the suburbs outside Boston, and planes are never watching. The police around here don't even use planes. So it depends where you live.

Similarly, you'll see electronic devices about the size of a small refrigerator. The device sits on the side of the road and monitors the speed of oncoming traffic, and displays your MPH in large lightbulbs. (Like this, only smaller.) Just like the "aircraft" signs, it's pure deterrence: The device has no camera, and there's never an officer nearby. The police set up these devices and just leave them for a couple of days. People slow down.
posted by cribcage at 10:08 AM on September 28, 2006

Pennsylvania frequently uses aircraft (fixed wing and helicopter) operated by State Police on both the Turnpike and major Interstate highways. Most drivers caught by them are either isolated on a lonely stretch or driving at a considerable rate of speed (>20 mph over the posted limit).
posted by flyingrock at 11:01 AM on September 28, 2006

look for lighter, small plaines moving parallel to the highway, and usually about 1-2k feet above.

That won't always be a cop however. Many pilots flying under Visual Flight Rules (VFR) will use landmarks such as roads to navigate.
posted by reverendX at 11:03 AM on September 28, 2006

As plenty of people have pointed out, radar is usually not involved. The point of visual speed traps is to catch the speeders who have radar detectors.

But to directly answer your question, it is entirely possible for aircraft radar to determine the speed of a moving object that is close to or on the ground. That's the whole point of military AWACS, and the technology is called look-down radar. I doubt that it would be an economical way to catch speeding motorists, though.
posted by randomstriker at 11:09 AM on September 28, 2006

It's simple trig if the speed and angles are constant.

No, it's not just "simple trig". From a moving aircraft, it is impossible to precisely aim the radar beam only at your moving target. You have to scan the area, which results in signals being returned by everything below you, including the ground. Your onboard computer then analyzes the signals to differentiate between the different Doppler shifts of objects moving at different velocities. Remember, the ground is also moving relative to the aircraft, which is why you can't just filter out the signals with no Doppler shift as with a handheld radar gun.
posted by randomstriker at 11:27 AM on September 28, 2006

They have those signs all over here in Nova Scotia, and the painted markings on the highways, and have had them for years. Imagine my surprise hearing a report on the radio last year of police plans to resume flights, which hadn't been flown for about a decade.

According to the cop interviewed, the stopwatch timing isn't even necessary - it becomes quite obvious from that height to spot reckless drivers or excessive speeders.
posted by GhostintheMachine at 11:33 AM on September 28, 2006

Their numerics aren't always accurate. There was a furor in Motorcycle forums because one of these aerial surveilances clocked a certain bike at 212 mph. The bike wasn't capable of such speed, and the poor bloke brought in dyno telemetry etc to the court to refute the validity of the claim (the bike wasn't capable of that speed so how could he be going that fast!)

Judge threw it out anyways...
posted by stratastar at 11:56 AM on September 28, 2006

At 2000 feet altitude above a highway, a pilot has about a 15 mile view of highway on a clear day. At 3000 feet and up, haze is the major horizon limit, except in winter, when the air is drier.

You don't have to be directly above the highway to see speeding cars quite well enough to time them against roadway hash marks, and no electronics are required, as it is purely visual observation. It's better in court than a radar stop, because it's direct testimony of the observing officer, and there's no machine certification to dispute. Most private pilots use straight sections of highway to stay in practice on maneuvers such as timed turns and S curves. Every student pilot learns them, using surface roads and intersections as markers, and many continue to practice them to maintain proficiency.

Interstate and defense highways are also built with the requirement that at least 1 mile in every 5 is built straight, to serve if necessary as an emergency runway, so setting up timed observation sections that will be accurate on major roads is easy. A visual observation of average speed over a measured mile against a stopwatch can easily be more accurate than a Doppler radar timing, and will already be as favorable to a driver as the instantaneous Doppler number on a rapidly decelerating car.

In Florida, one of the best uses of aircraft traffic observation is keeping drivers alert on hot summer days, when the long stretches of straight Interstate down to Miami, Orlando and Tampa induce sleepiness in drivers cruising for hours at highway speed. You'd be amazed at the number of people that doze off in the afternoon going 65 mph down I-95, and just drive off the road at the first slight curve.
posted by paulsc at 12:00 PM on September 28, 2006

I was going to say what SpecialK said: these signs are posted along I-5 between Portland and Eugene, and there are still cops everywhere. Those signs don't mean the area is a no-mans-land where you'll never be pulled over.
posted by peep at 12:01 PM on September 28, 2006

Interstate and defense highways are also built with the requirement that at least 1 mile in every 5 is built straight, to serve if necessary as an emergency runway

Snopes disagrees.
posted by DevilsAdvocate at 1:12 PM on September 28, 2006

I-75 between Tampa and Gainesville uses them. Have seen helicopters and planes used.
posted by 6:1 at 2:38 PM on September 28, 2006

On "VASCAR": http://www.radartest.com/article.asp?articleID=5500
posted by jepler at 5:07 PM on September 28, 2006

In Canada the white perpendicular marks on the highway they use for timing with aircraft are exactly 500m apart. So you can use them to calibrate your speedometre(requires stop watch) and odometre.

In BC you usually only see aircraft when traffic volumes are high. I've seen as many as two dozen cruisers working with a single plane. They actually built a special centre meridian pull out to go along with the marks on the highway at one location on the Coquihalla.
posted by Mitheral at 5:28 PM on September 28, 2006