[Publius6961]

"One solution which will never happen: Electromagnetic transportation. Same principle as the high speed trains, but everyone has to go purchase their own little "car". It has no internal combustion engine, just magnets. The individual can retain their own little space and privacy"

Cannons.
I've always been partial to cannons. With the proper planning, they would need to be spaced no less than twenty miles apart.

Smaller, 'feeder cannons' can deal with local 'traffic'.

[kanawa]

Cannons.

Sorry, they contribute to global warming/cooling.
The smart money is on slingshots.

[mbynack]

I prefer transporters and wormholes myself.