How many people in the US have simple access to a tornado shelter (either in their home or within short walking distance)?

My question is about people in areas frequently hit by tornadoes.

“One of the largest single-system tornado outbreaks in United States history occurred from April 14 to 16, 2011, resulting in confirmed tornadoes across at least 14 states and severe to catastrophic destruction on all three days of the outbreak. Current estimates suggest that at least 37 people have been killed from tornadoes and an additional five people were killed as a result of straight-line winds associated with the storm system. The outbreak of severe weather and tornadoes has led to 42 deaths in the Southern United States.”

http://en.wikipedia.org/wiki/Mid-April_2011_tornado_outbreak

Why do people with access to shelters do get killed?

No warning? Not aware of a warning? On the road with a turned-off radio? Not enough time to get to a shelter? Not enough time to get out of a car or truck?

Or is curiosity, like that of storm chasers, but without the proper knowledge?

What could be done to reduce fatalities and injuries in the future?