By making parking free for 20 mins this would:
Increase footfall for traders as customers arrive, quickly buy their things and leave.
Increase sales which would stimulate investment in high street by owners.
Give family businesses on high street a level playing field to compete as huge multinationals offer free parking in out of town malls.
Stop the victimisation by parking wardens of motorists guilty of shopping locally.
More things but I can't remember them now