If you are departing to the north, the flight is kept below arriving traffic. When the airport has landings to the west, which is most of the time, the inbound traffic from the northwest arrives over the Woodlands and travels over Porter to the north of the airport. Once you have traveled far enough north after takeoff, the flight is past this traffic and you are cleared to a higher altitude. If I recall correctly, the flights are held to either 3000 or 4000 feet unless there is no traffic. In my experience, this takes about 5 - 10 minutes after takeoff, if you are departing on runway 15 L/R and there are arrivals on 26 L/R and 27.