They have remained virtually unchanged. Simply flat surfaces upon which we operate a proliferation of vehicles. In the last few decades however there have been many who have come to question the viability of these roads and highways. Ecologically they do form firm physical boundaries that does prevent the unnatural migration of wildlife species. Ecologically these ever increasing boundaries are a disaster for maintaining natural specie diversity.
Specie loss is a certainty.
The return of this necessary connectivity in both wildlife and human habitat could be resolved and even enhanced with elevation, enclosure or burial. Unfortunately the cost of which has remained preventative. The anticipated cost of burying, enclosing or elevating todays conventional highways results in an exponential increase in the cost of structural outlays. In conversations on this wonderful web. I have found many who are concerned about resolving this looming ecological disaster.