Detours are inconvenient and time-consuming and sometimes mysterious, when drivers wonder why they're being forced to go miles out of the way.

Last weekend, for instance, the Minnesota Department of Transportation shut down westbound Interstate 94 between I-35E and Hwy. 280 in St. Paul. The posted detour took drivers north on I-35E to I-694, then west to I-35W and finally south to Hwy. 280 where they could reconnect with I-94. Or they could stay on I-35W and catch I-94 in downtown Minneapolis.

The I-694 route was much longer than having drivers cut across on Hwy. 36 or major east-west city streets such as Larpenteur Avenue.

So how did MnDOT choose it?

Ron Rauchle, a west area engineer for MnDOT, explained that city and county roads are not designed to absorb freeway traffic, and some state highways are not either.

"When we have a detour that is necessary due to road or bridge construction, we will detour traffic onto MnDOT roads designed for that higher volume of traffic," he said. "Keeping them on the state highway system is the preference."

In the case of the I-94 closure, that meant keeping freeway traffic on freeways, even if it meant a longer, circuitous route. "We detour onto a similar type of road," said Tiffany Dagon a MnDOT pavement engineer.

Rauchle said in rare situations, MnDOT will detour traffic onto a county or city street. But if it does so, the agency must pay the road's owner for extra wear and tear or for any upgrades that a city or county would make to accommodate extra vehicles.

In the case of northbound Lyndale Avenue in Minneapolis, where the bridge over I-394 is closed for repairs, diverting traffic into downtown was the most feasible option. True, that put additional pressure on a downtown grid already overtaxed with construction projects of its own and westbound I-394 traffic that was backed up because of repair work there. Northbound Lyndale is used by 31,000 vehicles a day, but it is a city street, not a highway. Thus, it was deemed that a detour using other city streets — Hennepin Avenue, 11th Street and Linden Avenue — was viable and the best way to get motorists to I-94 or Lyndale.

Steer clear of neighborhoods

"Even if people don't agree, we want to have the least amount of impact," Rauchle said. "We don't want to detour drivers onto a roadway that serves a residential function. You would not want to live on one of those roads and have traffic volume triple."

MnDOT has a traffic control unit that studies traffic patterns and helps plan detours acceptable for motorists, neighborhoods and cities. The unit also determines if and when detours are put in place, or if lane restrictions might be sufficient. Detours can add extra time and miles to a trip, Rauchle said, but they are planned to be the least disruptive to drivers and neighborhoods near construction.

"We don't just willy-nilly pound a stake in the ground and say, 'Here is your detour,' " Rauchle said. "We plan within our agency and with other agencies."

Yet not everybody will follow an official detour; they'll turn to navigational devices for a shortcut.

"It depends on where they are going," Rauchle said. "That designated state detour route might take them too far out of the way. They can find a shorter route."