I think it's to do with cars getting stuck inside them - it can be difficult for marshalls to extracate the car as they get jammed inside the 'cavity' (you don't want to get jammed in a cavity -
) York use motor-cycle tyres, cut in half around the centre circumference of the tread, then have a wood disc to cover the hole up - they make superb track markers.
I really enjoyed today, pity Colin is so fast -
Looking forward to the next meeting up there - August 23rd.