To make them efficient and reliable would be costly, with current tech.

Maybe some variety of maglev type interface between the ball and the chassis.

Why not make the road out of motorised rotating 'tracker' balls (MRTBs) which can rotate in the left and right, backwards and forwards directions and which could push any object which they detected on them in the direction that the user wanted to go?

You could surf along a road like that on a plank of wood provided that you had a control box to tell the balls that were currently under the plank which direction to rotate in.

If only the MRTBs which detected the weight of a 'vehicle' on top of them rotated then people could travel in different directions. This would be a form of transport that everybody could use and gets round the old problem of conveyor belt systems which only allow travel in one direction.

Hey, it could work like a computer mouse in reverse. Problem is the dirt which would foul up the connection or slipping in the motor. Also, the contact surface with the road would be small, so it would not be especially maneuverable.

IT would be a great idea.... the problem of not of not enough traction can be solved by having the ball actually be hollow but similar to michelin tire. it would have a little bit more contact surface between it and the road. also a major issue would be weather, if the ball was completely smooth it would just slid all around the road, maybe some sort of tread in the future could fix this.

connection to the car would be difficult, it would have to as said be a maglev or a type of roller/ball that controls it. the only problem with rollers would be the they cant go in all directions(only front to back/side to side) and being a ball it does not go in only one direction so the only answer would be to levitate the car over the ball some how.

steering it a difficult subject because of the lack of contact with the ball and would have to be done with a sort of gyroscope/magnetic force type device. as magnets on the sides and in the front and back would have to guide some sort of complex symmetry of magnets in the ball forcing it to go forwards backs wards or side to side. although it would interfere with the maglev idea it could work maybe in the future

A sphere is mathematically a point-contact, while a cylinder-tire is linear. I know that the pressure is regulated, so actually athe sphere would be a circle and the regular cylinder-tire is a rectangle, but in all cases of equivalent wheel size, pressure, etc, you'll get more road-contact with a cylinder than an ball.

Road-contact is generally good, it equals traction, which equals both safety and speed.

Also, balls sticking out the sides of the car would be detrimental to the aerodynamics.

Finally, the ball is going to require more material (rubber), hence more weight, even though parts of it aren't in use, ie contact with the road. Unsprung-weight causes a harsh ride and that extra rubber costs money.

And I echo all of a-ronn's comments about difficulty in engineering.

It just looks cool.

How would the city get back the money that they are going to loose on speeding tickets.