After a little research online, I found credible information regarding the effectiveness of the radar signs. As all other forms to stop speeders It has its pros and cons.
Pro's: Moderate cost. Highly effective in slowing traffic. Mobile.
Con's: Requires power (DC or solar).
They are called radar speed signs, speed reader boards, driver feedback signs and "Your Speed" signs. They can be permanently mounted like any other traffic sign or they can be attached to a trailer and moved from one location to another. All speed reader boards tell drivers their actual speed as they pass by. Some flash warnings when speeds reach a pre-set limit.
The studies done on driver feedback signs indicate that they are highly effective in slowing traffic, particularly when used on residential streets, near school zones and around playgrounds. They have greatest effect on those who are traveling significantly over the speed limit. Interestingly, research results also indicate that these traffic calming devices have a long-lasting effect. They continue to calm traffic even after they become long-standing fixtures at a location. A study conducted by the Texas Transportation Institute showed that almost the same speed reduction was being achieved four months after installation. Other studies have shown drivers exhibit traveling significantly reduced speeds even months after the sign are removed. In a recent survey, traffic engineers and other safety professionals ranked driver feedback signs as the most effective traffic calming method for neighborhoods and school zones. Researchers suggest that the sign's effectiveness is due to the fact that, unlike static speed signs that are often ignored, feedback signs refocus driver attention on their own speed rather than on their personal evaluation of driving conditions.
Radar speed signs do require power to operate. Some are designed with batteries that require ongoing recharging. Solar powered signs are also available.