Spatial (in)accuracy of cell broadcast alerts in urban context: Feedback from the April 2023 Cannes tsunami trial

Since June 2022, France is equipped with cell broadcast technology which alerts individuals within a predefined area. Despite the proven effectiveness of this technology, few studies take a spatial view of cell broadcast alert at a local level. Trials carried out in France were assessed only on their technical success, without verifying the rate of reception of the message by individuals in the official alert area, or the gap between the official alert area and the actual broadcast area. This study focuses on a trial conducted in April 2023 in Cannes (France). Using a geo-located survey method and spatial analysis tools, we show how cell broadcasting is more imprecise than one might think at the local level. Reception rates depend on the telephone operators and a large and ragged edge effect is measured, which means that the message is broadcast far beyond the area defined by the authorities. A second approach was to check the reception of three cell broadcast messages sent within a 20-min interval at fixed points, which revealed the fluctuation of the broadcast area over time, making its spatial extent complex to predict. Similar works should be carried out in other urban and rural areas.