Motorists will have to slow down this fall when stretches of I-15 are switched from 65 to 55 mph.

"The Wasatch Front is a victim of its own growth," said Kim Morris, spokesman for the Utah Department of Transportation. "Federal law mandates that speed limits in urbanized areas can be no higher than 55 mph."For all practical purposes, the speed limit on I-15 will be 55 mph from north of Ogden to Spanish Fork, with a few exceptions.

Perhaps most significantly, there are two chunks of freeway that now are considered "urbanized" - a stretch of road in from Draper near Point of the Mountain and another around the Kaysville-Centerville area.

The segment in the Kaysville-Centerville area will become 55 mph, and the same is true for the Draper area, except for a three-mile section that will remain at 65 mph.

Both routes are heavily traveled, and some commuters no doubt will be dismayed to have to lighten up that heavy foot on the accelerator.

There are other exceptions, too, such as 200-foot lengths of what would be considered non-urbanized areas like those in Utah County. But these are so short that it isn't practical to put up signs telling the difference, Morris said. "You really need about a 6- to 7-mile stretch."

The Census Bureau draws the urban boundaries, Morris said. "We have no say in where they are drawn. . . . We have until Oct. 1 to bring our speed zones into compliance. "

Morris said UDOT has yet to decide how to inform drivers of the change. The switch will mean new road signs or perhaps pavement markings, which he said have proved successful.

Morris said UDOT will meet the federal deadline but added, "There's also interest among Western states to lobby Washington to get the 55 mph speed limit abolished.

"We would rather have speed limits set by approved and practical engineering methods as opposed to what we consider an arbitrary speed limit," Morris said.