IF you are curious to learn more about the work of the traffic police, you now have the chance to see how one of the very first specialised units of the Hong Kong Police evolved. The traffic police unit was established early this century. ''The History of Traffic Police'' exhibition, which opened in July at the exhibition gallery of the Force Museum, will continue for about one year. Photographs of a full range of police motorcycles, from the classic designs used in the '20s to the latest models, are arranged in chronological order. Visitors can also see photographs of old police cars and road safety activities, as well as get a glimpse of the first road safety campaign held back in the '30s. Also on show is a model of a road safety city, a miniature of the junction of Causeway Road and Tung To Wan Road. Other objects on display include a radar speed check device used in the '70s and road safety campaign publicity material printed over the past two decades. The Police Museum, at 27 Coombe Road, is open on Tuesday between 2 pm and 5 pm, and from 9 am to 5 pm from Wednesday to Sunday.