The demand models (trip end, distribution and mode choice) produce 24 hour person trip matrices by purpose and mode in production/attraction form (trips are ‘produced’ at home and ‘attracted’ to workplaces, schools, shops etc). The function of the time period factors is to allocate the trips in these matrices to three time periods (am peak, interpeak and pm peak) and to convert them into origin/destination form (to reflect the actual direction of travel) prior to assignment to the networks. Additionally, for the road assignment procedure, we must convert person to vehicular trips.