But Harris said rapidly changing technology now creates too much uncertainty for that process to work well in the future, and planners are struggling to adapt and incorporate more flexibility.
For example, Blaine Leonard, UDOT's director of intelligent transportation systems, said Google promises to sell a self-driving car by 2020, and many other car manufacturers are working on similar vehicles — which promises to change transportation dramatically.
Harris said self-driving cars could boost the capacity of each freeway lane from 1,700 cars per hour now to about 4,000 — because such cars can safely follow one another more closely and avoid weaving and other problems that slow traffic.
"We could double the capacity" of freeways without any more construction, Harris said, although only some lanes likely would be reserved at first for self-driving cars, while other lanes would be needed for older vehicles without such technology.
While that technology could increase road capacity, it also could vastly inflate traffic demand.
For example, "If I do own a [self-driving] car and I don't want to pay for parking while I'm at Squatters, I set it on 'roam' " to let it drive itself around until summoned. "Imagine the implications" on traffic demand, Harris said.
Such self-driving cars also could allow people who are now too young or too old to drive by themselves to go on the roads while the car does the driving.
Harris said that increases the estimates for traffic demand from self-driving cars anywhere from a few percent to more than doubling over coming years — making planning difficult.
Even Uber and Lyft are creating more uncertainty.
Some people predict they could lead to the demise of public buses and trains, while others see them as leading to more transit use by making it easier to travel inexpensively from the end of a transit line to a final destination.
Harris said the services also make it difficult to plan how many people may actually own cars in the future and how much they will drive because many people may find using Uber and Lyft to be cheaper and easier than owning a vehicle.
Harris said all that is leading UDOT, the Utah Transit Authority and regional planning agencies to experiment with a new way of planning until the future is more clear: developing different plans for various scenarios.
For example, UDOT could come up with scenarios about how much freeway work is needed based on how much highway capacity goes up by autonomous cars.
UDOT, UTA and regional planning agencies in Salt Lake and Utah counties are working on a first-ever flexible plan, Harris said, with various scenarios as they look at the future of the Interstate 15/FrontRunner corridor in those counties.
The agencies are looking at how changes in highways, freeways, transit, trails and various new technologies could affect one another, and are developing scenarios for public consideration.
"When you try to create new planning processes, it's messy. And it is messy. But that's the fun of it. We're inventing things as we go," Harris said as agencies try to deal "with all the crazy things going on now."