Derren said:The players should not add to the game world. That is the DMs job.
The players might not be able to add to your game world, because you have decided that is your job and yours alone. That's not how it has to work, and I think for the majority of posters in this thread, it's not how we feel it should work either.
Further - why so hung up on whether or not a particular dead-end alleyway has a sewer grate or chink in the fence or other avenue of escape that isn't on the map or in your adventure notes? Is this alleyway crucial to the plot of the larger adventure? Is it someplace that you plan to revisit again and again? If it's a crime scene or the location of the entrance to a secret forbidden temple or something, perhaps so. But if it's just an obstacle in a chase scene or some other "throwaway" location, what does it matter? As an obstacle to be overcome, it should provide a moment of dramatic tension and then the scene moves on - perhaps complicated by the outcome of a die roll, perhaps not.
How does it hurt the game for the player to suggest some aspect of the scene you hadn't included, and then let the dice influence the outcome? Notice I said "influence" and not "decide" - this system seems to leave ample room for DM interpretation.