"Participants shall refrain from intentionally or accidentally blocking, charging, obstructing, or interfering with the forward progress of another participant. Any violation of this Section shall result in a variable time penalty."
An obstruction foul is called when one participant gets in the way of the forward progress of another participant. It does not matter whether the obstruction was accidental or intentional, because the effect is the same on the obstructed athlete.
For example, a runner who stops to tie his shoe on a narrow course might block or slow down a runner coming up from behind. That might lead to an obstruction penalty. Obstruction might be called on someone who stops or gets in the way in transition, slowing other athletes down.
The best way to avoid an obstruction foul is to use good manners and be sure to step off the course if you must stop.
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Have a question about the USA Triathlon Competitive Rules? Please contact Charlie Crawford.