United Airlines saw traffic climb in December, continuing a pattern seen for much of 2018 as the Chicago-based carrier has aggressively added new routes both domestically and internationally. But the aggressive growth strategy also had a somewhat negative impact on domestic load factors for the month despite typically heavy holiday travel. Per United’s December ops report released late Wednesday, United’s domestic traffic for the month climbed 5.5 percent year over year. The biggest boost came in regional traffic, which jumped 9.3 percent year over year as United extended its service to many more second tier markets serviced by United Express regional flights. Mainline traffic climbed 4.7 percent year over year in December. Increasing economic turmoil within China, a country where United has been trying to grow market share in recent years, could at some point begin to affect traffic on United's China routes.