We propose a novel approach for Ad-Hoc WiFi based distribution of information within large crowds of mobile users. The work is motivated by civil protection scenarios where infrastructure based communication often breaks down in cases of emergency. We follow a basic opportunistic networking ap proach by making use of the smartphones' built-in WiFi hotspot functionality which in combination with the devices switching between access point and client modes facilitates the propagation of messages on a multi-hop basis. We make three contributionswith respect to previous work on this topic. First, we empirically determine core boundary conditions given by the performance of modern smartphones. To maximize system performance under such circumstances we propose novel heuristics for a mode switching strategy based on client mobility instead of random strategies that have mainly been utilized so far. Finally, we compare its performance to a random role switching strategy in a large-scale simulation based on a real dataset consisting of movement traces from 28'000 people during a three day festival in Zurich. Within the simulation we investigate the influence of various parameters on the system's behavior. I. MOTIV ATIONManaging crowds at large scale events and during everyday life in urban areas is a complex and highly dynamic problem. Up to date knowledge about the current state of a crowd (e.g. with respect to its density) is absolutely vital for operational personnel. This includes the earliest possible detection of potential hot spots and the means to communicate with the crowd in an efficient manner to solve problems before they turn into critical situations.Over the past years, we therefore developed a smartphone app-based crowd management system, which analyzes smart phone sensor data voluntarily contributed by visitors of large scale events and creates a real time overview about crowd conditions such as crowd density, crowd pressure and crowd turbulences [1], [2]. More importantly from a communication perspective, the system also provides the means for event organizers and emergency forces to cOlmnunicate with the crowd in a targeted fashion utilizing location based messages [3].Until today, this system has been deployed during numer ous international events and was consequently used by over 100.000 users who contributed well over 50 million data points in total. The deployment during the Zurich festival [4] resulted in the creation of the -according to our knowledge -largest GPS based mobility dataset collected at a public event at that time.A key concern that has emerged during virtually all de ployments was the ability to deal with network outages. Such outages must be expected during emergency situations and are also not uncommon at large enough public gatherings under normal circumstances. Clearly, a network outage during an emergency would mean that the system functionality (sending messages containing safety advice) would become unavailable at the very moment when it is most needed. As a consequence we have ...

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