Equal Split in the Informal Market for Group Train Travel

In this paper we make use of a unique dataset collected in the central train station of

Kiel, Germany. A group ticket is used by individual proposers who search for co-travelers to

share the ride with shortly before the train departure. The bargaining behavior resembles

the Ultimatum game to the extent that proposers request a fixed price for a shared ride and

potential co-travelers usually accept or reject the deal. We observe that the prevailing price

corresponds to the equal split of the ticket cost between the maximum possible number of

co-travelers. This result is remarkable because the positions of the bargaining parties are

hardly symmetric and the formation of the full group is not guaranteed. Using a simple

agent-based model we are able to identify some sufficient conditions leading to the observed

distribution of prices. Finally, we show that the probability to accept an unusually high

offer is decreasing with the price and increasing when the offer is made right before the train

departure.