Park posted a 10-under 206 total, three shots clear of fellow South Korean Kim Sei-young (70), the LPGA Rookie of the Year, with compatriot Ryu So-yeon (69) and Spaniard Carlota Ciganda (69) four strokes behind.
Australian halfway leader Minjee Lee plummeted seven shots off the pace after a 75.
It is usually a player's responsibility to meet his or her tee time -- being more than five minutes late results in disqualification -- but officials decided to waive the rule due to 'exceptional circumstances'.
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The LPGA said the official tournament vehicle had taken more than two hours to complete a trip that normally takes 15-20 minutes, an excruciating journey that prompted American Angela Stanford to issue several tweets while she was stuck.
Park was on an earlier bus that, although also delayed by road closures and traffic, arrived well before her tee time, and she had no complaints about the situation.
"I needed a little more time, which really worked out well. It's better than rushing," she said.
Park said she again had trouble judging the distances for her approach shots in the thin air, but she more than made up for that by wielding a hot putter.
"It's really hard to get the distances right here. Even though I played really well today I didn't hit the iron shots really close.
"I made a lot of long putts today. The ball-striking wasn't A-plus but the putter was definitely A plus."
Ciganda, who was on the bus that experienced the major delay, described the situation as 'crazy' and thanked LPGA officials for showing leniency.
"It was too much traffic and we didn't know what was going on," she said.
"Angela (Stanford) called the LPGA people who were really nice because we could've been disqualified."
(Reporting by Andrew Both in Cary, North Carolina; Editing by Greg Stutchbury)
