My son has much more experience in these matters than me and last summer, we were cycling on part of the route about to be used in the TdF above Harrogate when a car drew alongside us and the front seat passenger threw at full force, a plastic bowl containing cereal and milk which hit him right on his side and covered him in the detritus the driver wanted to chuck at him! I was enraged that someone should do that to my son (even though he's a fully grown man, that protectiveness never goes away) and called them to come back and face me. They were too cowardly and zoomed off so we never saw them again. I'd taken their number and make of vehicle (5 series BMW) and informed my son I was calling the police but he told me not to bother. He'd had an actual fist fight with a car driver when out cycling one day and the police got involved but were worse than useless. They were obstructive, time-wasting and non-productive so following that, he would only get the police involved if it was very serious as he's found it more hassle than it's worth.