Just making it a bit more random, there is a 1 in 20 ticks chance it will be true.
It's the "random variance" I mentioned, if you leave it out, the cars will spawn as soon as the last one has cleared the spawner.
I'm not online much at the moment, I'm trying to finish my entry for the Tizen contest, and still have too much to do.
Edit: That's not an ideal way to do random, I was rushing a bit. The faster a computer processes ticks, the faster it will come up true.
In other words, faster computers would spawn cars faster.