Security-line times are notoriously unpredictable, but two American airports are making it easier to plan exactly how far in advance to arrive before your flight departs. Hartsfield-Jackson Atlanta International (atlanta-airport.com) and Denver International (flydenver.com) airports provide real-time updates on their Web sites, refreshed every 15 to 30 minutes. Hartsfield-Jackson will even send the updates directly to your PDA or cell phone, through its Trak-a-Line program. It’s worth noting that the TSA also publishes security wait times on its Web site (tsa.gov) for 450 commercial domestic airports, but the data is less reliable, as it’s compiled over a four-week period prior to your departure date—and does not reflect actual conditions.