How A Speed Radar Works on Motorcycles

Knowing how a speed radar works can protect you from an undeserved ticket

Understanding speed radars
Before you are caught in the radar gun sights here's what you need to know about speed radars.Photography by Dean Groover

Our reader’s e-mail sounded desperate: “I am absolutely certain I was not going that fast, but the cop showed me the radar and its read-out said 69 mph. Can the radar be wrong? Was there some weird electrical interference? I can’t afford the ticket or the insurance premium increase. What can I do?”

When I talked to the reader on the phone, I discovered that there wasn't much he could do at this point, since he had not observed what was going on around him and what other traffic was on the road when he was pulled over. He had seen the oncoming cop across the highway's divider, saw that he was staring at him as he went past and then watched him in his mirror with rising apprehension as the cruiser cut across the median and tore up behind him with his lights flashing. The startled rider knew he was being pulled over, but had no idea why. He thought the cop was joking when he told him the speed, since he knew he was sitting right on the 55-mph limit. But the ticket was no joke.

He thought there was something amiss with the radar. My guess is the problem was with the officer’s interpretation.

Understanding speed radars
If caught in the speed trap have the officer consider the other factors in the situation. For example, if there was there a semi behind you, if you appeared to be going the indicated speed, or if the semi was going just as fast as you.Photography by Dean Groover

What It Sees, What You Get
A number of years ago, six of us were returning to Los Angeles, California, from Daytona Beach, Florida, riding west across Texas on Interstate 10. In those days of 55-mph speed limits, we were going a bit over 60 mph. Our six bikes were gradually overtaking a lone rider on a Yamaha XS650 twin, and a semi-truck was coming up on us from behind, getting within approximately a quarter of a mile or less. As we approached a small rise, an eastbound police car appeared. It was alone in the left lane of the eastbound side, a classic trolling pattern for cops using moving radar. Our group and the truck slowed down to approximately 55. The Yamaha rider, who was perhaps 100 yards ahead at that point, did not need to slow down, since he was already at or near the speed limit. The police car hung a U-turn. When he passed the truck, my heart dropped, but he went by us too. Instead, he pulled over the XS650. I wondered if he'd flag us over as we approached, but the truck and our six bikes passed without attracting his interest.

The actions of the police car looked like those of an officer getting a hit from his radar. I saw the radar unit with something displayed as he overtook us to pull over the Yamaha. Therefore my assumption was that the radar’s message was the reason for the stop. Since he had no interest in us (and I would have expected him to if he was making a stop of one motorcyclist for some criminal concern) and did not act as though he was pulling the guy over because he had some more important mission, I am almost certain that it was a stop for speeding. It was a few miles before I realized what had happened. As the police car topped the rise, his radar unit locked onto a target that was exceeding whatever minimum lock-on speed had been programmed into it. Looking up, the cop saw that the nearest vehicle was the Yamaha, so that’s what he pulled over.

However, what he apparently didn’t realize is that the radar will indicate the speed of whatever target has the largest aspect—the vehicle that “looks” largest to it. Which is not necessarily the closest vehicle or the fastest one. If it “saw” our six-bike group as a single unit, it might have read us, but more likely it was getting a reading from the truck, which was also the only vehicle that was actually going fast enough to attract the interest of all but the most cite-hungry cops. The Yamaha was certainly not going fast enough to warrant a ticket if he was speeding at all. The cop just didn’t know what his radar saw and pulled over the wrong guy.

My bet is that this is what happened to our reader. He seems to be sure that he was not going over the speed limit. Unfortunately he doesn’t have a mental picture of what sort of other traffic was in the vicinity, so it will be difficult to fight his ticket.

What should you do in such a situation? Well, first of all, as soon as you realize you are being pulled over and that a radar unit might be providing the evidence, take a moment to look around and see what other vehicles are in the vicinity. This is true even if you think that you were in the wrong. Unless you have someone with you, your only collaborating witness will be the officer, so I’d try to get him to take note of and even record some facts: Did you notice the semi (or bus, motorhome, SUV or car) following me (or going away in the opposite direction)? How far behind me did you estimate it to be? Did I appear to be going the speed that your radar read? Did you notice how fast the semi appeared to be traveling? Did you notice that he was overtaking me? Did he appear to be within the range that your radar normally picks up such vehicles? Do you think your radar could have picked it up instead of me? Why? Would you make a note of those observations, please? They may prove useful in court. If you are calm, polite and earnest, you might even get him to realize that he may have misinterpreted the radar and let you go.

The metallic frontal area of your vehicle, especially vertical surfaces facing the radar antenna, are what reflects radar's microwaves (laser beams are a different matter, they don't care about metal) back to the radar unit, so your clear plastic windshield, leather saddlebags and Cordura-wrapped body won't read on radar.

What You Thought
Of course, he may have an attitude that motorcyclists are always speeding and that you are, therefore, the speeder. Such an attitude may have made him believe that he "saw" you speeding. Let me offer another first-hand experience about such perceptions. Back in the early 1970s, I was driving a borrowed Volvo 144 on an Illinois interstate, and was exceeding the 65-mph limit by a considerable amount. I had just passed a group of four big American cars as we approached the top of a rise and I reached the top just as I turned back from checking that I could change back to the right lane. Ahead in the median was a cop car. I made a quick lane change and got into the brakes very hard, slowing down below the speed limit. As a result, the four bigger cars re-passed me as we approached the police car. But it looked like I was busted anyway. The trooper pulled out after we went by and pulled over the first of the four cars, which I don't think was ever going fast enough to merit attention. Then he jumped out and started waving the following cars over as well. I was now last in line and turned on my signal, but was amazed when he waved me off, clearly signaling that I was not to stop.

Stunned, I continued on my way, at a significantly lowered speed. Eventually I realized what must have happened. After his radar locked on to me and he saw the speed, he looked up the road. What he saw was a slow-moving import being (or about to be) overtaken by big American cars. He probably had the attitude that Volvos were underpowered little cars that were driven conservatively and that the speeders had to be the guys in those big, powerful American cars. Even though my target was almost certainly the one the radar saw and he should have known it by looking (since I would have been in front when the radar alerted him), his expectations and attitudes said the other cars were the likely speeders. I would have loved to hear the conversations when he told those drivers what he clocked them at. If any of those people are reading this, I offer my sincere apologies.

The point is that he apparently made a judgment error based on his experience, attitude and perception. In this case it worked—very unjustly—in my favor, but it could have been four motorcycles I was passing too, and he might have pulled them over.

It would have been better for those guys if I had been riding a motorcycle that day, since a motorcycle’s frontal area is considerably smaller than a car’s, and the speed radar never would have picked me up while the cars were so close. I could have slowed down without the radar ever reading my speed. Let’s look at how this happens.

Understanding the speed radar with motorcycles
Even a relatively fat cruiser like the Road Star has a much smaller frontal area than the average car, therefore it has to be much closer to the radar gun before the unit can see it.Photography by Dean Groover

Size Matters
The metallic frontal area of your vehicle, especially vertical surfaces facing the radar antenna, are what reflects radar's microwaves (laser beams are a different matter, they don't care about metal) back to the radar unit, so your clear plastic windshield, leather saddlebags and Cordura-wrapped body won't read on radar. But your engine, frame, headlight and all those chrome parts bounce the radar waves back to the unit. If you have plastic parts or a helmet finished with metallic paint, they will reflect the radar signals back to the unit. Therefore a full fairing and saddlebags with metallic paint will increase your radar signature. A Gold Wing, for example, is a much more substantial target than a bare cruiser V-twin.

The fact that a motorcycle’s frontal area is so much smaller than a car has several implications. First of all, you must be much closer to the radar unit than a car before it will register your speed. A few years ago we conducted some tests with our speed radar gun. On an open desert road with a minimum of objects to disturb the radar’s beam, we found that while it could pick up a semi about a mile out and an SUV going away at over 4000 feet, it couldn’t pick up an approaching mid-size motorcycle until it was inside 900 feet (a sixth of a mile from the gun).

I recently found one of those trailers in my neighborhood that has a radar gun and a large digital display beneath a sign that says, “Your Speed.” I tried riding towards it on a Road Star at a low speed while cars were overtaking me in the left lane. Even when I was almost on top of the trailer, it almost never showed my speed, picking up instead the speeds of the more distant, but larger, vehicles. I went out at night when there was little traffic and noticed that it could pick up cars over two blocks away—on the other side on an overpass—but didn’t notice the Yamaha until it was half a block or less from the trailer. On one occasion, I thought it was showing an inaccurate speed, but the target turned out to be a car in a parking lot that appeared to be obscured by other parked cars.

The shape of the radar’s target also makes a difference. Using our own police-spec radar gun, we learned that a typical oncoming car was much closer to the radar gun before its speed would register than it was while going away. The reason is that the streamlined front of a car does not reflect as much of the radar’s signal back to the gun as the flat, squared-off rear. The flat rear of a truck, van or SUV is a perfect reflector. These vehicles’ speeds can be measured by radar as much as a mile distant on our gun if they are driving away from it.

We have also repeatedly seen a phenomenon that could spell trouble for motorcyclists. A reading for an approaching or retreating vehicle could blink off for a couple of seconds, then return. There are other documented quirks with radar guns, such as getting a reading from the patrol car’s fan, which can cause problems for all vehicles.

Combined Confusion
Put some of these situations together and you have the making of a potentially unfair ticket. Let's say the officer is sitting in his car. A van goes by on the other side of the road. Its speed comes up on the display: 62. That's above the 55-mph limit but not enough to get the gun to lock on at the 67-mph speed the officer set. He watches the speed increase until it's 65. Then the display blinks off. The van has apparently gone out of range. Meanwhile he has watched a motorcycle approaching on his side of the road. Then the display comes up again: 68. Bingo, the radar locks it in. The cop pulls the bike over. The only trouble is that the speed was the van's, not the bike's. Even though the cop waited for the van to apparently drive out of range, when the gun picked it up the second time, he mistook that reading for the bike, which was much closer.

From our experiences with radar, we would be reluctant to accept almost any ticket based on speed-radar measurement of a motorcycle’s speed if there is another vehicle in sight moving in a parallel direction downrange of the radar gun. I don’t think that an officer can factually testify that he is certain that speed shown was the motorcycle’s if there was another vehicle out there. If you get pulled over, look around and see what other traffic might have been in the beam with you. If there is, that should provide the reasonable doubt that saves you in court.

You can use this knowledge to get yourself in trouble too. A friend, who was aware that radar couldn’t read his speed when other larger vehicles were around, was passing a motorhome when a cop heading the opposite direction turned around and pulled him over. The discussion went something like this.

“Based on my radar reading, I am writing you for excessive speed.”

“But your radar could not have detected my speed when I was so close to that motorhome.”

“It didn’t, but I got the motorhome at 73 and you were going significantly faster than he was. I estimate you were going in excess of 80 mph.”

The ticket was for 75 mph, and it took the cop a long time to write it because of the details he had to include. My friend did try to fight it but the cop was persuasive in court and the ticket stuck.