I can understand how the holodeck creates virtual environments. But I don't see any mechanism that prevents people from walking into walls. Are the floors moving? Are they in a different dimension that has more space? Do they simulate the feeling of movement while keeping you stationary?
Some simulations appear to take up far more physical space than the holodeck would allow (as in people are physically all over the virtually created world - like Voyager's 24/7 running Irish city).