Mainly because cars of the period 1948-1966 (and a little give and take either side) aren't confined to West Sussex forward-parking fields, and neither are their period-dressed occupants. Have car, will travel, and so, every year, I find the party gets started on the A3, as soon as you leave the M25. Or, the M27 if you're coming that way, or the A27, if you're coming that way. You get the point.
And so, as ever, my top 10 cars on the A3 this year. Extra marks for what I could see of period-costume dressed occupants as I overtook, and a gold star for those actually on the hard shoulder of the A3, at such an early stage in proceedings.