Smart phones. Smart cars. Coming soon: Smart freeways

By Tom Herrmann / ADOT Communications

When a stretch of I-10 between Eloy and Picacho Peak gets a first-of-its-kind dust detection system in about two years, something else new will arrive with it: variable speed limits.

Where most speed limit signs show a number for the maximum speed, variable speed limit signs have electronic displays that allow traffic engineers to adjust for conditions. They look a bit like scoreboards used at high school sporting events, but without the clock or the visiting team’s score.

The concept isn't new. Variable speed limits appeared on the New Jersey Turnpike in the late 1960s and have been used in several states, often to adjust for weather or traffic conditions. Washington, Tennessee, Wyoming and Georgia are among the states using variable speed limits. In Arizona, Scottsdale has used them along a short section of Camelback Road.

Until now, they haven’t been used on Arizona freeways. That’s about to change.

Drivers have been encouraged to slow down when blowing dust reduces visibility to a dangerous level. In the dust detection zone between Eloy and Picacho Peak, variable speed limit signs will give that safety advice the force of law.

When dust detection equipment senses blowing dust, it will automatically activate the speed limit signs to slow drivers from the current 75 mph down to perhaps 35 mph. It won’t happen all at once: Speed limits will slow 10 mph each 1,000 feet to bring drivers to a safer speed. Traffic operators watching on cameras in Phoenix can return I-10 to normal speeds once the dust has cleared.

ADOT has been working with consultants, the Arizona Department of Public Safety and the Federal Highway Administration to research, design and build a dust detection system to protect drivers in the area that sees more blowing dust than any other freeway in Arizona.

Construction of the system, along with widening I-10 in much the same area, should begin by the end of the year.