Generate an random round race track with any given number of straights and 90° curves

i'm trying to build an small script to generate an random racing track that always connects to its start out of an fixed set of tiles.

Does anyone have an hint or an link to some algorithms that can help me with this?

For better understanding: 1. Give an number for available straight pieces 2. Give an number for available curved pieces 3. Generate an round course that always connects to its starting point without using more than the given tile numbers.

Sorry for my bad english. Thanks in advance for all your help. :)