The pic comes from here : http://ariel-motorcycles.jlbweb.fr/alb_ ... alogue.php
There is a second picture that could be from the same occasion, inside a hall with all (competitors?) lined up.
As it's a French website perhaps the pictures are also from somewhere in France, that would explain the lining up on the right side of the road.
Maybe someone who knows a lot about fashion could pin point the country or year?
Surely events like this where a novium in those days, there must be more pictures..............somewhere