Taking shelter outdoors
If the tornado is far enough away and road options and traffic allow, you should try to find a substantial building for shelter. Follow the basic tornado safety guidelines (get in, get down, cover up).
Motorists have found truck stops, convenience stores, restaurants and other businesses to be adequate shelters in a tornado situation. Walk-in coolers can sometimes make a good shelter.
While you should never try to outrun a tornado in your vehicle, you may, in some situations, be able to get out of the tornado's way by driving out of its path, or stopping and allowing it to pass.
Again, this can be extremely dangerous unless traffic, time of day and road options allow you to see the tornado, determine which way it's moving (and how fast), find a road option that will take you out of its path (while avoiding other storms) and to safe shelter.
The worst-case scenario for motorists would be to be trapped in your vehicle on the road with no escape possible. This scenario could occur in more densely populated areas, in metropolitan areas at rush-hour or in high-traffic situations, or on limited access roadways, such as interstates or turnpikes where it might not be possible to quickly find safe shelter.
It may become necessary in these situations to leave your vehicle and seek shelter in a low spot.
Highway overpasses are not tornado shelters, and these should be avoided.
Ditches, culverts, and ravines should be used only as an absolute last resort. You will be exposed to flying debris, rain, hail, lightning and extreme wind. People have survived by seeking shelter in ditches, but people have also died.
If you must leave your vehicle to seek shelter in a ditch, you should try to get as far away from the vehicle, as well as any other potential debris, as possible.