I wouldn’t even bother to wait for/ask for a dispensation in those cases. It’s simply prudential judgement to not risk your own life or that of others by being on the road in bad weather. Of course the definition of “bad” varies from place to place. A storm that dumps 6 inches of snow in Quebec where I live won’t have the same impact as it would say in Atlanta. We’re used to winter driving and all our cars are equipped with snow tires by law, in winter. But a mega storm like we had in March (which was on a weekday though), which dumped 30 inches in less than two days, is another matter, and for me anyway, ice is always a no-go.
That said regarding travel, you don’t need a dispensation to travel to an area where Mass isn’t available or if in transit in places (like airports, train stations) where Mass isn’t available. Again prudential judgement is required. We must do our best to recognize the special character of Sunday as a day devoted to God; when traveling I do that with the Liturgy of the Hours which I pray every day.
**Circumstances for a dispensation are those where you can reasonably attend Mass, but have some other event or reason to miss it. **One example in my case was when I was in a fundraiser bike ride for a cancer charity. It was a two day ride (Saturday and Sunday) from Montreal to Quebec City, about 145 km per day (90 miles). I asked for and received a dispensation to participate.