how do i design an equall lane length track

An odd number of bridges and constant lane centres will give exactly equal lane lengths.
(There's more explanation here on Chris Frost's web site )

If you don't want any bridges the lane lengths can be evened up by widening the lane centres to give the outside lane a short cut.
(There's more explanation here on Chris Frost's web site )

It is worth mentioning that equal lane lengths do not necessarily mean equal lap times, although with 2 lanes the differance in times would normally be small.

QUOTE (Abarth Mike @ 20 Apr 2012, 08:37) <{POST_SNAPBACK}>............ It uses standard commercially available plastic track pieces to design a track and it will give you lane lengths, but if you ask it to equalize usually it puts in a crossover.
The reason it puts in a crossover to equalise is that it is impossible to make the lane lengths the same with constant lane centres and no bridges or cross overs.