Uh, why? Are the Flagston kids prone to troublemaking by squirting water at each other thru straws? Or does Hi have the belief that the rate of urine production depends on amounts of liquid drunk, and he doesn’t want to have to stop again too soon? Or is there some folk theory under which hamburgers and water just don’t mix well?
Comics I Don't Understand 
I think it’s the fewer-pit-stops-on-the-road theory but I can’t be sure.
It almost looks like a middle strip in an arc, but I’m pretty sure Hi and Lois doesn’t do multi-day stories.
As @beckoningchasm suggests, there was a bit of a story arc, covering their tourism vacation. It is still ongoing at Comics Kingdom. There have been episodes of them staying in one crowded motel room, and visiting a variety of attractions. But the closest to answering the question posed was maybe this one:
Though this probably speaks more to worries of nausea going thru the rolling hills than to the lack of rest stops.
I think it’s clear that Hi’s primary objective was to minimize the frequency of pit stops. It’s worth mentioning that this strip was originally published in 1966: the federal interstate system was only about ten years old, and the infrastructure for travellers that exists today just wasn’t available back then.
P.S. B.C. (2) – I think you’re right that the strip doesn’t do arcs now, but like I said, this was nearly six decades ago.
Kilby, How do you find information about the original publication date?
@Chak; It’s right there in the copyright date of 1966 plus the 7/20 in the bottom corner.
It’s called sloppy hamburgers. Big hamburger with water dumped all over it. Makes the night so much fun.
I think it’s clear that Hi’s primary objective was to minimize the frequency of pit stops.
Yep, I think that’s most likely. But that opens the question of what factors control kidney function and urine production — if you’re eating, will reducing actual liquid drinking do much for delaying urine?
Also, does that mean that they would only have water to drink? And no juice or pop? From my point of view that’s just mean, dry eating. (But I’m not an impartial judge — I hesitate to start eating any hot-cooked meal without a drink ready-to-hand, ever since burning my mouth badly on a Burger King fish sandwich fresh from the oil and the soft drink not yet served.)
It’s probably pit stops, but with glasses of water and small children, there’s a large risk of spilling water all over everything.
Old enough to remember our station wagon with the extra storage thing on the roof; chaotic motel rooms; primitive seat belts; and the fight over who got to ride in the very back, stretched out amidst the luggage. In later years, my parents would say they trained us to have hundred mile bladders.
Is it just me, or do Hi and Lois — then and now — too often end a strip with one or both just looking tired and defeated, rather than comically flustered, or eye-rolling, or surprised, or any other non-depressing reaction.
Ah, 1966, the first and only time I, personally, had to pee by the side of a highway because of this very phenomenon.
Any traveling back that far in time involved me and both of my sisters in the backseat – the younger of the two a baby then – no seat belts in our cars then. I am guessing by memory and estimating the time, that the baby was 2 years old and it was our family trip to Washington, DC in 1966. Being girls (and traveling on Interstates) meant that there was no peeing on the side of the road – but I am pretty sure that the baby peed and more wherever she was. (She is now a successful business executive of some long time.)
Now I travel the same roads with my husband (no children and if we had any they would have children by now – and maybe even their children would have children by now) and we – okay, I, still have to figure out where the next rest area is and if I can make that far or do we have stop NOW. (In more recent years we travel in a Class B RV, aka van conversion, and we can stop anywhere and use our bathroom – though we try not to so we can dump as rarely as possible.)