Actually you are right, i just gridded this up on paper and at best all i could end up with is 1.5m (5 feet) lane width, although you could open up some of the corners a little more to allow for better passing room. Either way, it would still be rather tight even for 1/12 pan cars.
That said, i think you could do a field of 1/10th minis on it, christ the racing would be tight and passing would be hard, and it would seem really fast even with silver cans.
Sounds just like something i would like to do, here in Aus we are blessed with 200m + long tracks, 4.5m track widths and the like, 4 such tracks within an hours drive i where i live. What we dont have, is much of the way of indoor/carpet tracks, like in the UK, which are narrow and small.
@ OP, get out the grid paper and see what cool design you can come up with, make it as flexible as you can and even include an oval track within the design so you can change it up how you feel like at the time. Mini's, 1/12 pan cars and even 1/18ths would be suitable classes to run on it.