Speed signs on the side of the road aren’t built to the same tolerances and specifications as police radar guns so over time they may start to drift out of normal specifications.
Radar guns need to be regularly calibrated and certified, especially because people often challenge their accuracy in court.
Speed signs on the side of the road aren’t used to issue tickets so the rules are more relaxed. Because of this, they may drift in frequency outside of the normal accepted operating ranges, especially with outdoor temperature fluctuations or simply just with time. The sign may still display a speed, but your detector may not alert if it’s outside the scanning range of your detector.
Solar powered speed signs may start to lose power on cloudy days and with the batteries dying, the speed signs will drop in power or stop functioning at all.
I’ve seen many radar detectors alert to some speed signs and not others, and this is true with Uniden, Escort, Radenso, Valentine, etc. Countless other radar detector owners have reported the same behavior.
If you’d like to doublecheck your detector’s settings, most speed signs operate on K band so make sure you have your detector set up to scan all of K band. (Don’t use K band segmentation or any K Narrow options. You could also try disabling any K band filters as you test.) Some speed signs transmit on X or Ka band, but this is less common.
Radar detectors are designed to alert you to police radar guns. Just because a radar detector doesn’t alert to certain speed signs doesn’t mean that it’ll also have issues alerting to radar guns.
In fact I’ve seen certain manufacturers starting to intentionally add some filtering to filter out speed signs, but they don’t seem to be doing this too aggressively, presumably because of the constant worries of “My detector didn’t alert to a speed sign! Something must be wrong and my detector must be broken. I’m returning this thing!” lol
(On a related note, it’s interesting that people complain both if their detectors false too much, as well as not enough. It’s understandable, of course, but still funny.) ?