If you're running them as hill reps then run them at race pace or a session of:-
3x60secs; 3x45;3x30;3x15. jog backwards down recovery. All as hard as you can manage. Of course it depends on how long the motorway bridge is.
Otherwise, again depending on what you can do, run for 1 min up and down, recover for 1 min, repeat. 6 or 7 reps.
Any chance of getting away somewhere hilly for training?
Try starting in a lunge position, spring up (jump) swapping which foot is forward, land as you squat down. Repeat for i min or until you can't repeat again. (does that make sense? it's really hard and uses the muscles you'll use for hill running).
oh look, youtube has a video