Confusion over PSNI speed penalties - how much leeway is given if you go over the speed limit?

Yesterday on the show we were talking about speeding and you had been contacting us saying that you had been done for speeding when being just a few miles per hour over the speed limit. There seems to be some confusion around the 10% + 2 mph discretion that can be applied to motorist who are caught speeding. Stephen talks to Tim Shallcross, from road safety charity IAM RoadSmart and Stewart, his niece was badly injured by a speeding driver.