You would think the turnaround point would be a natural place for the race organisers to put a timing mat. They would capture an extra split time and be able to automatically identify runners succumbing to this temptation.
That didn't prevent the 2016 Philadelphia Marathon organizers from placing an unannounced mat at a turnaround point 22 miles into the race. It helped them catch hundreds of cheaters.
And even if you can afford the extra mats, another challenge is where to place them. It's obvious with an out-and-back course, but there are many shortcut opportunities on every loop path, and you can't place a mat at every apex.
It's too easy to match pictures to a bib number and then simply look at the splits and wonder why someone ran a record breaking 10 minute 5k near the end after cruising along at a series of 25 minute 5k's. An 8 year old kid can investigate this stuff.
That sounds like a nice and easy way to weed out a lot of cheaters - every race has one or two randomly placed timing mats.
"Fantastic investigative work. HOWEVER. Not sure why you had to do this. You see, I ran the race. And you wouldn't know this because you didn't, but there are four timing mats - start, finish, 10K and turnaround (about 9 miles)."
the answer from Derek is:
"I am aware of the timing mat at the turnaround. The timer did notice this, and she was initially DQ'd but was added back in until they went through everything. In every race there are missed mats - sometimes they malfunction, or they aren't placed across the entire course and are missed by an honest runner."