Procedural Generation of City Streets?

问题:

I've read up on L-Systems, read Parish & Muller and Kelly & McCabe, and tried implementing this, but I'm really not having any luck at all (all my streets tend to go in one direction, never branch, etc.). Can anyone provide some source code, or some sort of reference implementation that generates a system of City Streets (preferably along the lines of Parish & Muller, but at this point, I'll try anything). I've been working on this for weeks now, and getting nowhere fast.