Even though you're moving faster, your head isn't going to fall any further to the pavement on the highway than it would on a city street. Less abrasion at low speeds, but the bone breaking potential is still there.
With that said, a 3/4 would be a big step up in safety from a 1/2 without much added bother.
If the speeds do stay low, I wouldn't worry about finding a quiet helmet. Wind noise won't be bad at 60km/h so take the tradeoff and get a helmet with really good ventilation (and the best impact ratings you can find).