[JoeReal]

I'm having some concerns now about the ACC. I noticed that when I'm coming to those Speed Detector Signals on the road, from my far approach, especially when the Speed Detector is in the field of view of the Volt's ACC system, the Speed Displays are way higher than what my speedometer is showing. But when the car is an oblique angle as it approaches the detector, the Speed Detector Signal would display the same speed. And I was driving alone.

For example, the speed limit is 40 mph, and I am going 38 mph, the roadside speed detector would flash rapidly my speed as 55! Then as I approach the roadside speed detector, from about 250 feet away, the displayed speed falls back to 38 mph. This has happened in many many situations, with the worst speed display when the roadside detectors are on the outer curve so that it is in line of sight of the Volt's ACC.

What this means is that the ACC's interference is creating higher speed false reading on RADAR detectors. Am just glad that most traffic police are aiming their guns perpendicular to the cars, or from the back of the cars. There could be instances that some police would be pointing their radar directly on oncoming traffic as they radio the offenders to other patrols waiting up ahead. Or if you're following police with the Volt's ACC on, they might mistake you for approaching them at way higher than the speed limit, as they also have RADAR that points at the back and to the front to measure vehicle speeds of the cars before them and cars following them.

 

[Barry]

Interesting. Does it make a difference if ACC is not in use? It is possible that the detected speed would be lower than actual speed when driving above 55 mph.

 

[Tomt]

Police radar guns typically operate on the X band frequencies of 10.500-10.550 GHz. ACC radar usually occupies the band around 77 GHz. It's unlikely one would interfere with the other...

 

[porbund]

X Band is not used for radar detectors anymore, its usually Ka band or laser

 

[ClarksonCote]

As an aside, if police are aiming guns perpendicular to your car, the speed they measure will be zero. The radar detectors measure speed relative to their direction. If they aim directly at your car, approaching them head on, the speed measured will be your true speed. In between, it will be some amount less than your actual speed. Every radar in the world that uses Doppler can only measure the radial speed component, as it is the only component that induces a frequency shift.

 

[JoeReal]

The Roadside Speed Detectors could be using different band that will not interfere with the police's own but interferes with the Volt's ACC.

I know the effects are real, as I drive other cars. The 2006 Toyota Corolla Matrix that my son drives has no problem with these roadside radars as I have driven it along the same roads.

How about the Volt's Collission Detection System? Is it also based on RADAR?

I will try turning off the ACC on my Volt as I pass by these detectors next week and see what happens.

 

[JoeReal]

As an aside, if police are aiming guns perpendicular to your car, the speed they measure will be zero. The radar detectors measure speed relative to their direction. If they aim directly at your car, approaching them head on, the speed measured will be your true speed. In between, it will be some amount less than your actual speed. Every radar in the world that uses Doppler can only measure the radial speed component, as it is the only component that induces a frequency shift.

They can use infrared sensor for perpendicular speed detection (two sensor points), and not a problem of interference with the Volt's ACC.

 

[PilotAlan]

Short version: Don't worry about it.

Longer version:
Radar speed signs aren't calibrated. Ever.
Radar signs will show the speed of the biggest target it sees. Even if it's behind you, or going the other direction.
Many radar signs are not set to ignore traffic going away from it.
Unlike enforcement radar, they can't discriminate between mixed/conflicting signals, clarity of return, or opposing signals. When running radar, most of the time I didn't have a signal that was clean and clear.

The sign can't do any of that discrimination, it just shows the speed of the biggest target in its field of view, even if its a dirty, messed up return. They could make a sign that could do all that, but no one would pay for it.

 

[Dutch]

Short version: Don't worry about it.

Longer version:
Radar speed signs aren't calibrated. Ever.

This.
We have a sign like that near a school zone where I live. If it's just you then there is a pretty accurate reading. If it's you and someone else going the opposite direction in the other lane you get 99MPH flashing.

 

[rhodomel]

This.
We have a sign like that near a school zone where I live. If it's just you then there is a pretty accurate reading. If it's you and someone else going the opposite direction in the other lane you get 99MPH flashing.

I can understand when it gets crowded, if you're following another vehicle, and there is a gap, the radar reading suddenly jumps and it gets stable again once the car before you is out of the beam. The OP indicated that he's driving alone. I'd like to observe this with these radar speed signs. But the consensus it seems that with the standard equipment used by police, there's nothing to worry about. How about the camera speed traps? are they using the same radar band width? although there is no camera speed trap on the roads that I travel.

 

[Prohidium]

Is it possible that when it first detects something from a standby state that it is configured to display 55 until it actually has a proper reading? I think every time I have seen one of these things it is always high further away and only gets more accurate when you get fairly close to it. I rarely see them dead on to the speed my car says its going until almost within a few feet of it and by then you have gone past it. I suspect they are also normally high as they want you to slow down regardless of your current speed.

 

[saghost]

I think most automotive radar is continuous wave with chirping, though I've also read of phased array units.

The continuous wave radar is constantly giving off energy - when it returns, the radar unit notes the frequency shift and direction of the shifted return. Reflecting off an object moving at a different soured than the radar shifts the frequency slightly due to Doppler effects.

Chirping means that several times per second the radar introduces a frequency hop into the outgoing signal. By timing when those frequency hops come back in the shifted return, the radar can determine the range to the object.

I'm not certain, but I expect that the speed signs you're discussing are simple continuous wave radar with no chirp and no direction finding.

For your ACC radar to screw up the sign, you'd have to be transmitting on a frequency that's not quite exactly the sign's - off by a fraction of a percent so the sign would see it as a return at a different speed. I would think the chirping might confuse it, too.

If your frequency was that close, the car would see it as a return, too, though the lack of chirps should confuse it.

Pretty sure something else is going on.

 

[Steverino]

Is it possible that when it first detects something from a standby state that it is configured to display 55 until it actually has a proper reading? I think every time I have seen one of these things it is always high further away and only gets more accurate when you get fairly close to it. I rarely see them dead on to the speed my car says its going until almost within a few feet of it and by then you have gone past it. I suspect they are also normally high as they want you to slow down regardless of your current speed.

I think I have experienced this. It's like the sign is calibrating downward as it closes in on the speed. Or, I have never experienced this and just sounds truthy enough for me to think I have.

 

[scoops]

I've come across half a dozen speed signs in my Volt. I am almost always running with ACC on. Never had any problems.

Most of the signs are in slower speed areas. And almost all of them read 24mph when I have ACC set to 25. Not sure if the delta changes with higher speeds.