the main strength of the ptal approach is that it is easy to understand unlike more sophisticated modelling approaches ptal allows for direct data visualisation using a contour map where patterns of public transport provision are clearly shown figure the formulae take into account the walking time from specified points of interest to all public transport service access points saps ie bus stops rail stations light rail stations underground stations and tramlink stops within a defined distance threshold this is combined with the average time a user must wait for the next service several parameters define the walk catchment area