Complex behaviors are difficult to evolve. There is unlikely to be a "stay away from roads" allele, while a "no tusks" allele was already in the elephant population.

If they live in an area where there are lots of roads everywhere, completely avoiding them may actually be maladaptive, as not every road crossing results in a fatality and they could well be trapped in a small area without it.

If they live in an area without many roads it probably doesn't really matter.