Why does the vehicles Air Conditioning only work when driving on the highway?

Your fan is probably not working correctly. When you are driving in the city, not much air is flowing freely through the nose of your car into the radiator, etc. When you are on the highway, there is a lot of air flowing in. Has your car been overheating or getting hot?

Your A/C belt may be loose and/or refrigerant may be low.

Well! for me it sound like the fan is not working, and specially at idle the condenser it get very hot. the best way to check the fan is to start the car and open the hood and with the a/c on the fan should be on. If the fan is not on... check the fan relay or the fan it self. other problem could be the office tube, for example if the high side in the a/c is under 200 psi and the low side is highter than 30 psi the office tube could be the problem in must GM cars.