[dvd3500]

I "liked" two posts as they cover 96.53464% of what makes a nice track.

I would add to the comment about bridges/fly overs to not be afraid to go multi-level.
While you do have to take all the things such as linesight and marshalling into consideration a lot of places allow for multi-tiered layouts that are easy to see round and marshal.

For example, more often than not you will be up against a wall. Using the wall to have 2 straights on top of each other can add a LOT of space.

Also take levels into consideration when trying to get a long straight in: Running diagonally is more space them along an edge....

Digital racing adds more considerations...