Its Switzerland, so I'd assume the mountains and snow play a big part in increasing the cost of surface road building and snow clearing, making tunnels more attractive.
It is in the northern part of Switzerland, the very lowlands of the alps. In my two winters here there would be a few weeks of mild snow, at the very most in this area (and the first winter here had a 1 in 10 year blizzard).
And there are some topographical changes, but nothing very steep. There are 100 KPH highways all throughout this area, and more convincingly, fairly high-speed rail too.