We investigate how an ad hoc collaboration between devices which happen to be physically close to each other can improve the quality of pedestrian dead-reckoning (PDR). The general idea is that whenever two users come close to each other, their devices use the proximity information to improve their PDR location estimates. In a public space the improvement will not only affect the two involved users, but also all the other people that they will meet and collaborate with in the future. On data collected from the mobile phones of 12 users over a course of three days during an open air festival in Malta (a total of 60 walked kilometres) we demonstrate that such collaboration can improve the localisation accuracy by a factor of four and prevent unbounded PDR error. The results imply that collaboration in crowded public spaces enables even simple smart phone-based PDR systems to provide effective localisation over long time periods and distances.