Swiss roads are excellent, well maintained and well marked.
Swiss roads can be smooth and straight(ish) or they can be like a corkscrew.
For example, from Interlaken for the Berner Oberland to Luzern (Lucerne) the road has a few tunnels behind the Brienzersee, then around Brienz it is one or two lane and reasonably straight. A bit further it is down to one lane, and it climbs up the cliff face over Meiringen with many hairpin turns and a corkscrew tunnel. Then, over the pass and it is gently down with fab view over the lakes and just a few more hairpins, all single lane. Then back on the flat and you get an extra lane from time to time and a dozen or so more tunnels, then onto motorway past the mountains near Luzern and into the city tunnels. Then you have to park the car. Although not cheap, I have found that parking in the multi story car park at the station works well. Then onto the trains and boats and trams of Luzern.
By train, the carriages have big picture windows - even in the lowest class - with views of the lake Brienzersee because you go right along the shore, then past Brienz you cross the plain to Meiringen. On the way you can see all the waterfalls in the cliff sides and at Meiringen you can go up the funicular to the middle of Meiringen Falls where Sherlock Holmes battled Moriarty. Then, having changed ends (the engine goes from the front to the back and the train go off in the opposite direction) the train climbs the cliff face using the cog mechanism which allows very steep climbing without spirals or switchbacks. The views are incomparable. Up into the woods, and look at all the waterfalls from a different angle, and the lakes and you can touch the pine trees. Heading down the other side it uses the cog again, then runs between the lakes and mountains all the way to Luzern where it ends next to the ships for Lake Luzern (Vierwaldstaettersee).
Or the first part, the Brienzersee can be done by ship to Brienz where you join the train to Luzern. The train and ship stations are about 50 feet apart.
The best bit is you don't have to worry about humping your luggage. You can drop it off at your starting station and it travels the network and goes to your next overnight destination, leaving you just carrying a small bag with what you need for the day. The Swiss Travel Service does all the transport for you. You just have to relax.
Either car or train, 18 days in Switzerland, provided you stay a reasonable time at each stop will be magic.