my local indoor offroad track is all 4' wide lanes. the track is smallish, no real straights and lots of turns so speeds are kept low (even brushless wont get faster than 30mph)
this technical track is a lot of fun, and the lowish speeds keep bad crashes to a minimum.

with 1/8 scale, id think 8' lanes would be a minimum. if you have the room, no reason you cant do 10' wide.

one thing I knew I would do if I had the space to build a track......id have the inside of the truck basically flat, so that I could change the lane layouts and id have wood ramps that I could move around and change the track.

it takes a lot of work to build a good track. but IMO even the best track will get old after a while. so id want the ability to change the track layout. you don't have to have the whole layout change.....just having a long straight made that you can add/remove a chicane can make a big difference. same with adding a small ramp right before a turn or right before heading onto a long straight.
im jealous....I soo wish I had some land to build on.