Absolutely – I’ve done it a million times. But you should not approach it as a cold call. Write a letter or email to the person in charge of advertising, and give them a fact-based case of why the sign would benefit them (traffic on the highway, price, etc.). Then follow up to see if they received your proposal and get their feedback. If you cold-call them, they will tell you “no” just to get you off the phone, without even considering your idea. If they don’t respond to your calls, take it to the next level. I have had success mounting my proposal on “foam core” [buy this at Office Depot or other] and dropping it off at their office. They can’t ignore something that comes on a giant sheet of foam core, and their initial impression is that it must be something they ordered, because who else would go to that much work?
I had a similar situation with casinos on some billboards on I-20 in east Texas. I hit the Shreveport casinos. Initially, I just got one casino. Once I had the ad up, I sent photos of it to the other casinos to see if they wanted to give this one casino a monopoly, or if they wanted to also reach this traffic. Eventually, I had every casino in Shreveport on the billboards – and they’re still there today, 20 years later.