Take frequency of transit service into account.
Frequent bus routes are more useful then buses leaving once per hour. I know you assume "that you will time your journey in a way that you don’t have to wait for your first transport option" but that's not always realistic.
Maybe with coloured shading you could distinguish what is accessible on frequent networks (leaving at least once every 15 minutes) and what is accessible with less frequent service.
yeah, it's about averages not about ideal time
Transit frequency is taken into account for the different times of day and different types of days in a week (see Mapnificent Settings in the top bar).
The problem with exponential distribution is that it assumes independent events and I don't think catching the next bus is really an independent event (you choose the connection with the least waiting time, connections may wait on each other, etc). It's difficult, but frequency is taken into acocunt.
Great app. Thanks for putting it up.
Transit frequency/wait time greatly affect travel time. Please do try to include it in the app if the data are provided by the transit agencies.
I don't know how well versed you are in probability, but the standard way to model this is with the expectation of a negative exponential distribution: http://en.wikipedia.org/wiki/Exponential_distribution