Really?
So, you stopped every few kilometers to check out every single thing? You stopped at every gas station, rest stop, roadside attraction, town, village, city, and house along the way to interact with the people there and discover what you could learn about all these things?
Or, did you speed past 99% of these things at highway speeds, stopping only when you needed to, until you reached Indianapolis, you goal at which point exploration actually began?
Even speeding past a lot of things at highway speeds I was still seeing a whole lot of scenery, geology, landforms, and vegetation types I'd never seen before. Never mind the central USA ain't the west coast when it comes to weather, that's for sure!
Since then I've done that trip four more times, never using the same route to get there* - once via Vegas-Denver, once via Montreal, and once via Orlando - and it's felt like exploration every time. Even more so when I've got lost: I saw far more of Atlanta than I wanted to while searching for my hotel there, on the Orlando run.
Would I have seen or experienced any of that had I flown there? No.
* - coming home is different; I've a set route and just book it in order to make the last ferry on day 3 and thus not have to pay for another hotel night.
So, in what way were you "exploring" the path from your initial point to Indianapolis? This is the point that several people have glossed over. "Oh, sandboxes are all about exploration". Sure. Exploring the set locations that the DM has provided, "strewn" across the map. I.E. entirely reactive actions taken by the players in service to the material provided by the DM. And that material will never be objectively created by the DM. It will always be some interesting thing for the players to interact with - and "interesting" will be 100% determined by the DM's understanding of the group and his or her personal biases.
And yet were I to describe in some detail what the PCs see and experience during their otherwise-mundane travel I'd be accused of slowing things down and adding in too much no-stakes colour.
Setting design is IMO entirely the purview of the DM. Also, the bolded isn't necessarily true: while I put interesting-looking things on the map I also put a lot of mundane things there as well, and for all I know the players might decide that something I see as mundane - such as some little farming village somewhere - is of great interest to them.
An example from right now in my game: some time ago I stuck the little logging village of Esbow in a forest not all that far (about 4 days walk) from the big city that has slowly become the characters' de-facto base. Nothing more than a way-station along the trade route, really...until last session when a PC decided* to set up her base there. Suddenly, what I saw as a mundane little village is potentially going to become important, meaning we'll all be paying a lot more attention to it.
* - more or less, they're still in the field right now and that player won't be here next session, but the sense I got was it'll be a done deal as soon as the PC gets there.