421 is a very busy highway, so they get a lot more customers. Probably a few hundred per hour during peak time. If they make a profit of just $0.01 per gallon then they're doing just fine.
Compare that to 18 that has probably 1/1000th the traffic. Gas stations probably have 10 to 20 customers in an hour during peak time. But they still have to pay the bills, too, so they have to profit more on each customer.
It's not greed, it's just economics. If they don't make a profit then why would they be open?
Further, it's well known that Murphy's sells high ethanol gas that gives you fewer miles per gallon than other stations. I suspect that Speedway is the same, since they have the same price. So they pay less per gallon than other stations, and see you more often.
Even further, think about it this way.
You spend $0.20 per gallon more, that's a cost of about $2 to fill up a normal tank. So you spent $2 for the convenience of buying gas closer to home, not putting your life at risk from getting on 421, or the headache of dealing with people that can't drive.
