I've been thinking a lot about Highway lately and realizing what a smartly designed card it is. Generally I'm down on cards that only activate in the presence of resources that very well might not be present elsewhere in the kingdom, like +Buy. But Highway is beautifully designed in that, yeah, it does need +Buy to fully activate, but it's often worth it even without it. In other words, Highway is balanced at $5 whether or not you're able to unlock its true power, which is no mean feat of game design.
Here's my reasoning. Without +Buy, Highway is roughly a Peddler. At a more standard cost, I'm convinced Peddler would cost $4, just because of how all the Peddler-with-a-bonus cards (Treasury, Market, and Bazaar) cost $5. Some of these bonuses, though, go unused: Market is sometimes worthwhile even if you don't use the +Buy. Treasury is sometimes worthwhile even in the greening stage. Bazaar is sometimes worthwhile even if you don't draw it with two terminals. Admittedly, you usually want to make use of the bonuses on all three of these cards, and they're relatively weak uses of $5 if you don't. But in any given actual game, they might still be worth buying.
Thus, Highway, the rough equivalent of a Peddler with no bonuses, is situationally worthwhile at $5. And actually there is a small situational bonus anyhow, in the way that Highway allows you to do funky Remodel tricks on the low end and expand the reach of Smugglers, Talisman, Feast, University, etc, on the high end.
So I'm going to go ahead and assert that Highway is weak and situational but perfectly balanced at $5 in the absence of +Buys. When there ARE +Buys, however, then it really pops. But does it really pop in a way that Bridge doesn't? Bridge is a $4 card, and it can still be spammed (with Villages or TR/KC) to dramatic effect. So why would Highway be underpriced at $5 if it can do the same thing? As a non-terminal, it's easier to spam, but it doesn't provide $, and you need to collide your Highway stack with your +Buy source to activate it. $5 still seems like the right price.
As I say, an amazing feat of game design.
Edit: Ninja'ed on many points. That's what I get for being longwinded.