I think I may be disappearing into a black hole !!!
You are calling simstep on an array, not an instance of RoadVehicle. An array object doesn't have a simstep method defined, hence the compiler error.
1.test if the queue is full
2. if it is remove the first vehicle
3. if the queue is not full, get a random vehicle from a list and add it to the end of the queue
4. then print a summary of the queue and the last vehicle added.