Ultimately I think it'll have to be smart highways. You could embed a coded marker in the centre of each lane which the car can pick up using RF (e.g. an induction loop). This plus other constraints like GPS, visual-inertial odometry, etc should be enough to localise and remain safely within a lane boundary even if it's invisible. This would work fine through a layer of snow - some of the more expensive ski jackets have built in loops for detection in case of avalanches.

It's hard to see how we could get from where we are right now, infrastructure-wise, to there, at least here in the US. We can barely keep our roads driveable in some places, much less modernized with the latest in smart beacons and such.

It wouldn't necessarily need to be in-road, but there are numerous places where you could adapt. For instance, cats-eyes in the UK (retro-reflective markers) are placed on virtually all motorways at lane boundaries and the car could look for a signature from them (I imagine at night, autonomous cars already use them as a feature for lane boundary detection). Nobody bats an eyelid about re-setting cats-eyes every time a bit of road is replaced. You could also put boundary markers on either side of the road, in the central reservation or something similar.

The idea would to put the smart tech in the car. What's going at the roadside could be made very cheaply and integrated into existing features without needing to rip up the entire road.

Other countries have toll highways which are well maintained, that's another option for installation.

I don't see this happening in rural/country lanes, the cost would be obscene (but again, we do liberally use reflectors in the UK). I also don't see this as a panacea, it would simply be a backup in horrendous weather.

I've seen those cat eyes before, but never in locations where plows have to travel. They are also relatively easy to place, just requiring some tar and a manual "pick & place". Making something plow proof would require embedding it in the road, either with a drill or a "rumble strip" like divot.

Yes. Smooth, flat, pothole free tarmac would be nice.

Volvo has proposed driving magnetized nails into pavements in Sweden as lane markers. Not only are these useful for self-driving cars, they're good for snowplow guidance.

How often would the magnets need to be replaced?

As often as the asphalt topping.

Uh-oh. In other words, there's a smart road ahead, you can 100% trust it. Except when it's not there, because it hasn't been maintained for decades, and it's unclear where the missing pieces are. Can't see what could possibly go wrong.

Plus there's also no incentive for anyone to waylay the smart road off the physical road, because there are no evil people (i.e. with motives that are incompatible with the vehicle's survival).

It's a hint, like lane lines. You're going to be using GPS and LIDAR as well. Yes, some of the lamer self-driving systems put too much faith in lane lines.

Indeed. Which means that we're back to square one, "it's where most people are, which is exactly the spot where it's least needed; whereas in places where more conventional navigation fails, there's minimal incentive to also build and maintain this."

TL;DR: good roads will be getting better, bad roads won't.

Seems like self-driving cars will turn into trains and trams by another name.

If it can take you 90-95% of the way, and seamlessly pass over control for the non-beaconed start and end stretches, that's still a serious convenience boost over a train/tram.

I would love this! I love trains, we should have never stopped building them.

There's some interesting recent work at (IIRC) MIT using ground-penetrating radar techniques to image subsurface features of the roadbed in real time, as the car drives over them. Rocks, rebar, and such are almost completely weather-independent (short of torrential rains that wash out the road itself), and they don't change significantly over time.

Indeed!

Since off-road use for smart cars is comparatively small, we should optimize the roads to be smart-car friendly.

Perhaps we could install systems which would enable smart cars to stay in lanes, perhaps with some hardware preventing the smart cars from going off-road.

And while we're there, we could also have systems providing charge built right into the road! Wouldn't that be neat?

The lane-keeping and power systems could allow regular cars to operate on the same roads, for backwards compatibility reasons.

Sounds too expensive to build? Not until you realize that you've re-invented the infrastructure for trams and trolleys put in place a 100 years ago.