The image to the right depicts the same scenario.  However, because there is bright sunlight behind the runner, he casts a long shadow.

Upon seeing the runner's shadow, the guard detects the runner earlier, giving him more time to react, improving his chance of successfully intervening.

The image to the left depicts a man in a suit with a briefcase running along the front of a wall - unaware that he is approaching the edge of a cliff.

Positioned just behind the wall, in blue jeans, is a guard.  His job is to prevent the man from falling over the cliff, but he will not see him until he reaches the end of the wall.