ORIGINAL: oldschoolracer

My own personal experience with 1/18 offroad is that lane with is usually somewhere between 4 and 6 feet, oval tends to be wider.
Personally as far as making a layout my advice would be to not make it too busy, too many turns with too many jumps so you're not wrecking all over and having to run and flip a car over all the time.
And if it was me instead of trying to build jumps out of soil which end up being poor angles and hard to repair, to make temporary jumps that you can remove at the end of the day out of plywood and astroturf. That way you can return your yard to some semblance or normalcy when it's not being used for a raceday. And that way it's a little easier to tryout what works and what doesn't and rebuild them as neccessary.
I've raced a lot indoors and that's what they use on carpet offroad tracks, and I can tell you the jumps are better and way more consistent.