The ridge in the middle can either be scraped off, as previously suggested, or rolled. It is only raised because the bits either side have been compressed by vehicular traffic. You could top it with scalpings, and then roll it, but beware of the weight because of the culverts you mention.
Laying tarmac is a waste of time, unless you are prepared to dig out the old road and lay a proper base. That way lies bankruptcy!

JM