Cool and good link - had a very brief scan through it.

I think usually L systems are state free so that they do not have the historical background about other branches in order to do the collision detection. Maybe some are state-ful so that they can.

Although its possible to come up with some cities and towns that have some elements of real towns from the use of L systems and voronoi patterns I strongly believe that aside from a few rare exceptions (and those being more modern anyway) that cities are more complex than can be generated from either of these two mechanisms. Trees are very close to L systems and large scale interstates are close to voronoi patterns but cities, esp old ones, have cul-de-sacs, dead ends, and any number of weird goings on that is not suitable for a one rule model.

If I were financially independent or retired I definitely love to have a go at getting together a model complex enough to build realistic towns and cities. I'm still glad your giving it a go tho just to see how far it can be taken.