Fuel and food cost more at an interstate exit than they do in the town a few miles further along. The gap is consistent enough to be a pricing structure rather than an accident.
Exit land is scarce and expensive
An interchange creates a small ring of parcels with direct highway access, and there are only ever a handful of them. Everything built there competes for the same few sites.
That scarcity shows up in what the ground costs to buy or lease. A lot fronting an off-ramp trades at a clear premium over an equivalent lot on a town street.
Rent is a fixed cost that has to be recovered across whatever the site sells, which for a fuel station is gallons and drinks. Higher ground costs lift the price of both.
The customer is effectively captive
A driver who has already left the highway is committed. Rejoining and trying the next exit costs time and fuel, and the next exit is unlikely to be cheaper.
Retailers price against the alternative a customer actually has rather than against the cheapest option in the region. At an exit, that alternative is the station across the ramp.
This is why two stations facing each other can sit within a cent of one another for months. They are competing with each other and with almost nobody else.
Brands price by site, not by chain
Most fuel stations carrying a national brand are run by independent operators who set their own pump price. The sign overhead is national; the number beneath it is local.
Each operator prices against local rent, local wages and whoever is visible from the road. Head office supplies the fuel and the branding, not the daily figure.
Which is why the same brand can be among the cheapest in a town and among the dearest at the interchange six miles away, with no contradiction involved.
Why prices fall a short distance away
Drive a few minutes into the nearest town and the customer base changes. Stations there sell mostly to residents who buy fuel every week and know what it should cost.
Repeat buyers punish a price rise by going elsewhere next time, so margins compress and stay compressed. The pressure comes from familiarity, which passing traffic does not supply.
The distance needed is usually small. A mile or two of surface road is often enough to leave the interchange pricing zone entirely.
Planning stops around the pattern
Stopping is not optional on a long drive, so the useful question is where rather than whether. Fuel, food and rest do not all have to happen in the same parking lot.
A tank filled in a town before rejoining the highway costs less than the same tank bought at the ramp, and the detour rarely adds more than ten minutes.
The exception is a stretch with genuinely sparse services, where paying the exit premium is simply cheaper than the risk of running the tank down looking for better.