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Session 12 Staffan Algers
- 11. Changes of headway M9 (air) and B7 (rail) Headway of M9 is reduced, for ride times, 30 – 90 min . Samkalk produces much larger change of CS
- 12. Main mode concept - consequences We want to evaluate the effects of introducing high-speed rail between B and C for travellers going from A to C via B. The change in consumer surplus according to Vips is large but zero according to Samkalk. The reason is that Sampers does not take into account the possibility to go by air first between A and B. Since air is not taken into account when rail is subject to change, Samkalk cannot calculate the effect.
- 13. Main mode problem – Example Luleå-Örebro Vips gave a change in CS by 2 500 and Samkalk by 2 only. Only night train Luleå-Göteborg, via Örebro is taken into account. The combination airline Luleå-Arlanda, Arlanda express train Arlanda-Stockholm plus the rail lines Stockholm-Örebro is not taken into account. Evaluate faster train Stockholm-Örebro
Notas do Editor
- Vad gäller beträffande Vips? Är användare av Vips övertygade/säkra på att de resultat som Vips ger är"reasonable". Om dessa användare tycker det kan det ju bero på att de gillar Vips-resultaten bättre än Sampers-resultaten. Vilka empiriska data finns till stöd för att det ena eller andra systemet ger säkrare resultat?
- 1) bullet 1: Is this really correct? Sampers applies a structured approach first allocating to main modes and then between different lines. 2) bullet 2: For allocating demand between main modes Sampers uses etc...while Vips uses ride times and prices specified per mode and line to generate demand per line and mode 3) bullet 3: Is this not only for CS calculation? If so it should be stated clearly
- bullet 1: It seems to me as if the same information about headway and time components is input to bothe systems. The difference lies in the way allocation between modes and lines is carried out; There is no explicit assumption in Sampers/Emme that time tables are not used; what is actually done is that an allocation mechanism is used that under certain circumstances may operate poorly for time table based travel bullet 2: I don't see why this is correct given that I know also price and ride time. I would then choose between different lines at a certain stop incorporating the my GC including time and fare bullet 3: Route =line bullet 4: It does not violate the assumption; rather the approach may be a less satisfactory approximation of behaviour given the assumptions about traveller behaviour
- Note that the entire discussion on this slide is based on a) the assumption that a misspecified logit model is applied; there may be other variations of the logit model that are applicable and that also give logsums that are correct estimates b) The discussion here deals with choice between public transport lines; the headline seems to refer to more general situations
- This slide seems to imply that Vips gives the correct estimate and that the ratio Samkalk/Vips give a true measure of how much Samkalk overestimates CS for a headway change. However, since Vips is based on the RDT approximation, that we know is not a perfect approximation for long distance travel, there will also be an error in the Vips estimate the magnitude of which we do not know.
- 6th bullet: Alternative formulation: The logit model that is currently used in Sampers (average time model) gives logsums for combined transport service that may be a less satisfactory approximation 7th bullet: Very definite formulation; consider something milder e.g. "Samkalk calculation of CS may be erroneous when headway is changed"; Are you absolutely shure about the sequencing error? Consider less definite formulation here too.