Generally, the signal propagation delay is d/c. d is a distance between a transmitting antenna and a receiving antenna. c is light speed.
However in NS-3, there is Random Propagation Delay Model
Random Propagation Delay Model: The propagation delay is completely random, and changes every time the model is called. All packages, even those that are sent between two fixed nodes, experience a random delay. As a consequence, the order of the transmitted packets is not maintained.
What kind of communication environment does this model take into account?
I think the obstacle, etc is considered. But because I can't find any official document or paper, I'm not sure about this.