Make yourself as flat and as small as possible in that ditch, if that's all that's available. Keep your helmet and riding gear on for additional protection from hail... if there is not a building nearby for shelter. Stay away from tall people aka: lightning rods.
I must disagree regarding highway over-passes as tornado shelters. The National Weather Service advises that these locations experience extreme and often deadly wind conditions should a twister pass close in. Hail and heavy rain, you'd be pretty safe. Climb up the slope and hang onto the steel structure. You know, where the homeless guy is sleeping.
Tornado? If possible, ride the other way and quickly. Take shelter in the first available large, solid structure you can find... school, police station, brick out house, etc. Forget the bike 'cause if you and Toto get swept away, well, you won't be needing that bike anyway.
We have a saying in Texas:
"If the good Lord wanted you to live in a trailer house, He would not have invented tornadoes."