SALTESE, Mont. — Dr. Chris Servheen spends a lot of time mulling a serious scientific question: why didn’t the grizzly bear cross the road?

The future of the bear may depend on the answer.

The mountains in and around Glacier National Park teem with bears. A recently concluded five-year census found 765 grizzlies in northwestern Montana, more than three times the number of bears as when it was listed as a threatened species in 1975. To the south lies a swath of federally protected wilderness much larger than Yellowstone, where the habitat is good, and there are no known grizzlies. They were wiped out 50 years ago to protect sheep.

One of the main reasons they have not returned is Interstate 90.

To arrive from the north, a bear would have to climb over a nearly three-foot high concrete Jersey barrier, cross two lanes of road, braving 75- to 80-mile-an hour traffic, climb a higher Jersey barrier, cross two more lanes of traffic and climb yet another barrier.

“It’s the most critical wildlife corridor in the country,” said Dr. Servheen, grizzly bear recovery coordinator for the federal Fish and Wildlife Service, of the linkage between the two habitats.