You keep using that word. I do not think it means what you think it means.
Ahhh....my misspent youth...
1) Do units in the dike road hex fireing accross the dike get the +1 firepower penalty?
2) Is the +1 fireing-accros-the-dike-penalty cummulative? That is - is fireing accross two dike roads +2? Is fireing accross a dike road at a unit in another dike road hex behind the road a +3?
It would be cumulative, but I don't believe it will ever really happen in the current game scenarios.