As road networks become more congested, those tasked with the management of traffic flow and road user safety are turning to Road Weather Information Systems (RWIS) to collect the meteorological data they need to keep the traffic moving. Road tunnels pose a unique set of problems to highways engineers and management personnel as they are affected by the weather at either end and can have their own internal microclimates. Couple this to the added risk of pollution from stationary vehicles or fire after a crash, and the need for a sensitive and accurate sensor to measure visibility and the causes of reduced visibility becomes absolute. The RWS-30 has been developed to meet the specific needs created by the tunnel environment and complies with current international guidelines as defined by the PIARC Technical Committee.