Hey, Dawn, nice to hear from you, again! Although the Yellowstone and Grand Teton NP road distances don't look like a big deal on a road map, getting from place to place in GT and YNP will take you a lot longer than you think. For example, on a nice day with no traffic, it's a good 5-1/2 to 6 hr drive from Jackson to Tower Junction in the north district of Yellowstone. And that's with no/minimal stops. It also takes at least 2 to 2-1/2 hours for an experienced mountain driver to get across the Beartooth Pass via the Beartooth Highway without stopping. In July the BH is mobbed with people, cars and motor homes and most have never driven on mountain roads before. So few people drive over 30mph because of all the hairpin turns on the switchbacks on both sides of the Pass. Here are a few other things that you may want to consider. 1) July is at the peak of the summer visitor season 2) the maximum speed limit in Yellowstone is only 35mph, so getting, from place to place will often take longer. Animals can also be crossing/blocking/wandering along roads with hoards of people stopping to take pix, often, without warning. So, driving in the Parks can often be a bit frustrating and time-consuming. Ya gotta drive defensively, all the time.
I know that this might be a once-in-a-lifetime trip for you and that you want to make the most of it, Dawn. But if you want to create beautiful and exciting memories of your trip, without having any regrets, I highly recommend that you choose just one Park to visit this year. That way, you can book up on the Park's attractions and hiking trails now and then explore the Park in depth this summer. This would give you a great reason and an incentive to plan another Park trip. BTW, the city that I think that you were referring that's on your way to Glacier is Great Falls....not Grand Falls. Sure don't want ya to get lost. I know that you're going to have fun, Dawn, where ever you go....safe travels!
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