If the car was overtaking you then it will show the car breaking the limit, if it was just oncoming then it will be you that has caused the camera to flash. If it is you and you are sure that you were not speeding, then when the letter from the police arrives write a letter asking for the date and results of the last callibiration test prior to the date of offence. Don't be put off when they say they can not do this and just pay the fine - which they will, all you do is say that you will go to court and request the photo and test reults, failure to provide you with these will mean the case is thrown out by the judge.