WASHINGTON (CNN) -- The U.S. Department of Transportation is currently reviewing how the nation's airlines define and report flight departure times, and the accuracy of the times reported.

The DOT tells CNN the study began in April and is expected to wrap up by September. This audit follows a study of flight arrival times conducted earlier this year.

According to DOT spokesman Jeff Nelligan, safety on runways and tarmacs is also being examined as part of the audit. According to DOT, "runway incursions" are up 70 percent over five years. That includes planes colliding on runways, planes colliding with ground support vehicles, and near misses between planes.

The department is looking at ways airlines report departure data. Nelligan tells CNN airlines have different definitions of departure time, from the time the airplane pushes back from the gate to the time it actually takes off. He says DOT wants airlines to tell the agency how they're reporting departure time, and if it is recorded manually or by computer.

"The Report on Air Carrier Arrival Data," resulted in airlines reporting how they define arrival times, and if they use manual or computerized methods to record their data. The study, published in March, found airlines have five different ways of defining arrival times.