Power in the wrong hands: devices for changing a red light to green are great for officials responding to an emergency. But now the technology is available to anyone with $1,500.

• Author: Sundeen, Matt

To any motorist who has ever been stuck at a red light, it may seem like the perfect dream. You pull up to the intersection, you hit a magic button, the light turns green, and you go on your way. No traffic backups, no long frustrating waits, easier commutes, everyone benefits.

The truth is buttons that change red lights to green are not only real, they are widely employed by state and local governments. They are "signal preemption devices," the most common form of which is a mobile infrared transmitter (MIRT). When activated, a MIRT emits a coded frequency to a receiver in the traffic light that instructs the light to either remain green or change to green for the oncoming vehicle. The signal also changes the light to red for all other directions.

For law enforcement officers, fire departments and medical personnel, this control can dramatically improve emergency response. Signal preemption allows responders to move safely through busy intersections and save valuable time on their way to incidents. In emergency situations, even a few minutes can determine whether a criminal is caught, a building burns down or a life is saved. A survey by the U.S. Department of Transportation in 2002 found that in the 78 largest cities in the United States, 381 transportation agencies had equipped almost 27,000 intersections with signal preemption capability.

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Although signal preemption has many benefits, state and local officials are increasingly alarmed about the public use of the technology. MIRTs are now widely available for sale on the Internet...