It is the safest means of transportation in Phnom Penh. You will find our airport taxis lining up just in front of your arrival terminal. Airport taxis are part of the association of taxis, ensuring both a standard price and a quality of the service.
 
For more information on transportation in the city, check the following websites:
http://phnompenh.gov.kh/en/visitors-tourism/transportation/
http://teangtnaut.org/city-bus-lines-stops/
 
There is also a transportation information counter at arrival terminal.  There you can get a ticket according to your tariff destination.