You’ve seen them on trailers next to construction zones on the highway, permanently affixed to traffic poles near schools, and perhaps on the street next to the office.

Radar speed signs are a fairly common sight on highways and byways. Their purpose goes without saying: Hey buddy, slow down, will ya?

But do they work? Do drivers really slow down when they see them? Or, like many public radio pledge drives, are radar speed signs ignored by most?

A few traffic studies have examined the immediate and ongoing effects of radar speed signs on motorist speed. Generally, a radar speed sign will slow traffic down somewhere from 1 mph to 11 mph compared to conditions before the sign. Fast-moving highways seem to see larger speed reductions than easy-going side streets, which makes intuitive sense: The same light tap of the brakes to slow you from 70 mph to 60 mph on the Blue Route produces a far more modest speed reduction when you’re going 27 mph on Pine Street.

As you would expect, not every motorist slows down because some sign tells him to. One South Dakota study showed 74 percent of motorists sped before a speed feedback sign was deployed. Afterwards, the number of speeding motorists fell about 20 percent, enough for a 4 – 5 mph drop in average speeds.

So while the department can’t say for certain that a radar speed sign worked on any specific driver, they can say the signs are “slowing down traffic in general.”

The signs work without enforcement – some studies have shown an even greater impact when the radar feedback signs also photograph and ticket speciously egregious speeders.