Creating an interactive simulation of a large urban environment populated with virtual humans poses a number of interesting challenges, ranging from how to initialise the virtual humans in the correct locations to maintaining a real-time simulation. When considering a large environment it is beneficial to investigate automatic techniques that create and simulate virtual humans using the available computing resources effectively. In this paper, two contributions towards the population of a large environment are described. The first presents two methods that use an automatic analysis of the urban environment to determine the required population densities throughout the scene. These methods ensure that the virtual humans are distributed such that main high streets consistently exhibit a higher number of virtual humans than other areas. The methods presented here achieve above 90% correlation to the predicted population densities for the environment, which outperforms the state of the art. The second contribution concerns the optimisation of the two methods to facilitate their applicability to large environments. The methods presented are modified to insert virtual humans dynamically into the behaviour system when required. The approach limits the required computing resources whilst achieving above 75% correlation to the predicted population values.