While hiking in the Torrey Pines state reserve a few days ago, I saw a volunteer "guarding" the entrance to the southernmost trail (which was taped off). She said she was there to physically prevent runners from ruining the newly paved hiking trail (which, apparently, they continue to do).
She explained that the park is required to widen the hiking trail and repave it in order to be ADA compliant. In order for the new paving (an organic compound of some sort, not horrible asphalt) to set, everyone must temporarily stay off the trail.
Apparently some runners continue to break through the taped off trail, damaging the newly paved path. Park staff must then tear it out and start all over. "It's costing the taxpayers a bundle," said the park volunteer, who also noted that the constant tear-it-out-and-start-all-over process has put the improvements way behind schedule.