The roads around COMs are shut about 10-15 minutes before the end of the game, depending on how many are leaving. Therefore you'd have to leave 20 minutes before the end of the game to ensure your taxi could get through, and even then, the taxi may not want to risk getting stuck.
There may be some taxi's already parked up, but again, match traffic and closed roads mean that you could get in one, and just get stuck.
Once the roads are opened, taxi's do come in and pull up outside the main reception to pick up people, but these aren't ordered, they just pull up. All local taxi companies won't take let you book a taxi to pick you up from COMs until about 90 minutes after the final whistle.
Best bet is to walk to town. It's 20 minutes, it's guaranteed as you don't get stuck in traffic or roadworks.