A sensor (20, 100, 200, 300) for detecting the approach and exit of a vehicle (42) to a tunnel (46). In one embodiment there is a sensor (20, 100, 200, 300) with a photodiode (22a) that receives light through an inclined channel (30, 30') or aperture (32, 332, 432). The channel provides a field of view...http://www.google.co.uk/patents/US6243002?utm_source=gb-gplus-sharePatent US6243002 - Tunnel sensor
