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I'm reading a paper titled "A Simultaneous Bus Route Design and Frequency Setting Problem for Tin Shui Wai, Hong Kong" (W.Y. Szeto, Yongzhong Wu) which attempts to model an improved bus network in my home town. In page 8, there is an assumption listed that "the passengers go to their destinations using minimal transfers" which I doubt.

I look up the reference lists and eventually found out this saying originated from a paper titled "The allocation of buses in heavily utilized networks with overlapping routes" (Anthony F.Han, Nigel H.M. Wilson), in page 229. It mentions that:

Step 1: choose the route with minimum number of transfers (most transfers are made at major transfer points); if there is a tie go to step 2, otherwise classify as captive flow and stop.

(step 2 omitted as not relevant)

These two path choice criteria are a good approximation of expected user behavior in a congested, overlapping network such as Cairo’s. Because of the level of crowding on most buses, and because of the extra fare required on each vehicle boarded, passengers will try to avoid transfers where possible.

... Also, because there are a large number of routes each with relatively high frequencies, it is unlikely that a multi-vehicle trip will be faster than a single-vehicle trip for movements having both options available.

However, the condition that "because of the extra fare required on each vehicle boarded, passengers will try to avoid transfers where possible." is obviously false in Tin Shui Wai and it specifies that free interchange is possible, also the routes involved have frequencies up to 12 minutes headway. This may lead passengers to change buses even when there is an option without transfer to save in-vehicle time.

So, is the model flawed?

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The assumption for that paper is clear.

In reality some people may or may not use a minimum transfer route as they might need to do some shopping on the way to work or home for example.

That does not change the quality of the assumption used in the paper. Many simulations use assumptions to enable a solution to be found.

An initial model may be developed with a minimum transfer route then further tested with other scenarios such as: 50% of voyages have one transfer prior to final destination.

An assumption does not invalidate a model - assumptions are applied to enable reality to be modeled, such as assumptions for fluids when solving the Navier-Stokes equations in turbulent flow.

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  • $\begingroup$ The assumption is created in a situation (transfer fare, high frequency) which is the opposite of where it applied (free transfer with not-so-high frequency). Does it invalidate the assumption? $\endgroup$ Jun 11 '20 at 8:35
  • $\begingroup$ @MichaelTsang each transfer causes a nontrivial chance of delay and therefore to missed connection. And yes the assumption that fare causes this is quite untrue in many places. Most of northern europe for example has unlimited changes as the ticket is valid for a time not fare. $\endgroup$
    – joojaa
    Jun 11 '20 at 9:40
  • $\begingroup$ When the frequency is high, the chance of delay is insignificant (e.g. when transferring between 4 segments of 4-minute headway buses when compared with a 20-minute headway option without transfer but with 20 more in-vehicle minutes). $\endgroup$ Jun 11 '20 at 9:57

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