2. Trashing, live-draw, or sifting. For big Hwy turns to work, you really want to increase your odds pairing up your Hwy's & +buys/gainers and not getting stopped dead by greens & coppers. Light trashing is perfectly acceptable - you don't need an ultra-lean deck or solid engine. A few (two - three) Warehouses can be great for enabling big Hwy turns, light trashing (particularly non-terminal like Loan) is more than enough to enable big Hwy chains, and as noted draw makes it even easier to line all this up.
3. In line with the need for draw, City is strong combo card for Hwy. You can mix in a few terminals (Smithy, Oracle, Horse Traders), they tend to start at +2 actions/+2 cards, and with the right setup you can quickly get them fully stocked where they provide plenty of money & the +buys to pile the Duchies.
Quoted this for emphasis.
On a kingdom where Highways are strong I generally find it comes down to two things:
1) Who can get more of the Highways. Getting 4 of 10 Highways is almost useless for a megaturn.
2) Who can draw all their Highways on one turn.
In some ways, I'd say that while +buy or +gain is necessary to make a Highways decks work, you also need either trashing or drawing or it never clicks into place fast enough to be worthwhile over a money strategy. A Highways deck with weak +buy and good non terminal +card or good trash is going to be stronger than one with strong +buy and terminal +card or weak trash. Or to give an example, Highway + Herbalist + Scrying Pool and Highway + Herbalist + Chapel are going to be stronger than Highway + Bridge + Courtyard, imho.