Flow rate: estimating the numbers at the Oxford Race for Life

We were supporting at the Race for Life and were debating the number of participants. I timed a minute and counted the number of rows of runners who went past. Each row was roughly 5 runners wide. About 60 rows went past - so about one per second. That's 300 runners a minute. It went on for 10 minutes which means 3000 runners. The next day the Oxford Mail was saying "more than 2500" so it turned out to be a reasonable method.