I think the dispensation comes in when you are technically able to attend Mass, but part of your travel plans include activities with family or say an organized tour where attending would considerably disrupt the event and cause inconvenience to others who may not be Catholic.
I personally consider at least once a year a vacation trip can fall under “need” as we all need recreation and vacation to restore ourselves. This past summer, we traveled to Scotland and the nearest church was about 40 miles away on narrow roads, many of them single-track, where we stayed on the Isle of Skye. We had driven 8 hours to our destination the previous day, and I was too mentally tired the next morning to undertake the drive. Instead we went on a family hike which required a lot less concentration! (keep in mind I was driving a manual-transmission car with right-hand drive, on the opposite side of the road of what I’m used to). So I used my judgment and didn’t go to Mass.
But I do think the Church takes to legalistic an approach. I think it would be better to allow it to be a matter of prudential judgement (and would ease the load on priests as well), to say that habitually missing Mass for spurious reasons is grave matter, but to occasionally miss it for family or travel reasons, isn’t, besides the non-travel “grave reasons” that we all understand.
But then I’ve always been a “spirit of the law” rather than “letter of the law” sort of guy.
Don’t take this as official Church teaching, it isn’t. It is my opinion and you can feel free to flame away!