That's not true. The max speed of a bear approaches 40. They can cover 50 yds in 3 seconds, but probably not sustain that to cover 100 in 6 seconds. 

Furthermore, if you've noticed, Alaskan bush is not straight line territory. There are a variety of obstacles that inhibit a bears movement. 

Furthermore, if you've noticed, bears don't really go full-tilt assault with no warning. When I hike and I get a bad vibe, like the brush seems like good bear country, or the trail is overgrown, or i'm on a game path, or I hear some sounds or see scat, I carry the spray in my hand. 

You should read the story about the largest brown bear ever shot. Dude had to reload a clip from a rifle while it was charging. He was a ranger too and knew what he was doing. A single .45 bullet can drop a bear if it's a miracle strike, but the reality is that there is lots of statistical evidence showing that bear spray is more effective. Believe it or don't.