My husband is an architect. He is asked to go into schools and do a project called BOOM TOWN. A Boom Town is a town that has grown up quickly around either a natural resource or a supply (gold). Think of gold towns in the west. He discusses where things go and why. I have designed roads that can be printed out by the schools on cardstock. The kids have a ball laying them and adding their buildings. We push all the desks together and cover them with brown paper. Then I free form a lake and rivers from blue poster board. I crunch up brown paper for mountains. I bring in all of my son's matchbox cars. He made towns for them everywhere we travelled with a roll of brown packaging paper and markers.
I was in a doctor's office a while back. There were two little boys waiting a long time for their mom. I got some paper and made some make shift roads for their matchbox cars. They thought this was great. So, below are the roads AND a bridge. You can copy and paste these and make them full page to print out on heavy cardstock. Get a gallon size storage bag--and off you go with your little ones and their cars. Have fun.