Answer to Question #10996 Submitted to "Ask the Experts"
The following question was answered by an expert in the appropriate field:
Speed minders are being placed in residential neighborhoods to slow the speed of drivers as they navigate their vehicles, and thereby increase public safety. These speed minders apparently emit radar for 250 yards according to manufacturers. This 250-yard distance (according to one manufacturer) has been loosely verified by residents in the neighborhoods in our community. For homeowners within the line of these radar emissions and projectiles that continuously aim in the direction of the approaching (potentially offending) vehicles, there are numerous homes and homeowners who are, in effect, in "harm's way" of these moving bursts of radar. Can you provide more information as to how these radar emitting devices work. As a precautionary principal, what can we do to protect the residents of our neighborhood who are subjected to these devices 24/7? One gentleman just had brain surgery as is especially fragile.
I assume that you are referring to radar speed signs that measure and display the speed of passing vehicles. As with all radar sets, the devices transmit radiofrequency (RF) energy. They calculate the speed of passing vehicles from the very small change in frequency of the signal that is reflected from the vehicle.
Radar speed signs were made in the 2000s by a small company called Speedminder, which is now defunct. Perhaps your city is using older equipment made by the company, or you use the name speed minder in a generic way to refer to signs made by another company.
Virtually all radar traffic signs presently on the market, as well as those formerly made by Speedminder, are Part 15 devices that operate in the K band of microwaves (around 24 GHz), which is used by many radar systems and some communications applications. "Part 15" refers to a section of the FCC (Federal Communications Commission) regulations that applies to "unlicensed devices", i.e., low powered devices that people can use without a special license from the FCC. Such devices are limited to very low output power, a few milliwatts for radar speed signs, and are inherently incapable of producing RF exposure levels that exceed U.S. or other international exposure limits. Because the intensity of the beams falls off as the square of the distance, the RF exposure levels at typical distances that a person or motorist will likely to be from the signs will be a very tiny fraction of the exposure limits, and in all likelihood will be lower than RF signal levels from other environmental sources including cellular telephones carried by motorists or cellular base stations in the vicinity.
In short, I am quite sure that the RF exposure levels from the traffic signs will be far below accepted safety limits, even to someone walking close to the signs.
To the extent that the traffic speed signs persuade motorists to slow down and comply with speed limits, they provide significant benefits to the community, which is a fact that should also be considered.
Kenneth R. Foster
University of Pennsylvania