Ok......
Provided your bridge is level enough, AI should manage to cross it once you have set the bridge route as 'roadway'.
To do this, select the cells that the bridge route covers and then go to the Flags Tab......Enter Expert Mode.......then check the roadway checkbox.......finally press the Change button.
You need to do this AFTER setting the default flags, if you do it before setting default flags the bridge route will be reset to 'not roadway' again.
In MOST cases AI will now cross the bridge.
Getting the bridge pieces to the same level in-game is a trial and error process when the bridge is covering some steep slopes.
I think it is best to do the checking in-game using Normal Detail setting.
If I find a way of calculating this beforehand I will surely let everyone know.
Planck