It's interesting to see Volvo investigating the use of in-road magnets to guide self-driving cars. It would absolutely make it easier (and probably safer) to develop self-driving cars, but it will never catch on.
Nobody is going to buy a car that can only drive on certain roads where magnets are present. Similarly, nobody is going to bother installing magnets in roadways unless there's cars that use them. The only way to get mass adoption of self-driving cars is if they can operate without extra infrastructure.
This could possibly be useful, however, to improve positioning accuracy on busy, high-speed highways. But the cars will still have to be able to rely on other mechanisms once they leave those roads.
Magnets in the road could some day help ensure that self-driving cars stay on the right track, according to Volvo, which has just successfully completed a research project into their use.