The placards are also used in Canada and I believe in the U.S. as well when transporting dangerous goods. One of the main reasons you have to display the placards on your vehicle is for first respondents, they can quickly identify what if any possible immediate dangers are present in the case of an accident or perhaps a fire that is close to a parked vehicle. A lot of people don't know this but it's actually against the law not to display the placards, there are however a few under 100 kg exceptions here in Canada.