Should you warn others of the presence of speedcams at certain places? It might save them from getting into trouble with the law, but it might mean they might speed and get into accidents elsewhere instead, harming others and themselves?
Would it be better to let them get caught naturally by speedcams? Would it be even better to keep changing the positioning of speedcams, so that drivers are cautious everywhere as they can’t be sure if they are being watched?
When it comes to observation of the precepts, whether there is anyone ‘watching’ or not, the law of karma naturally ‘registers’ everything done intentionally. Karma-cam is everywhere and always turned on! As they say, take care of your karma from moment to moment and your karma takes care of you!