Transportation, Mexico City
About public transportation you have to be careful with Taxis, Subway and bus,NEVER take too much money with you, just the necessary or hide it.
About food, never eat on the streets and be careful with vegetables, some stablishments dont desinfect them properly, like lettuce.
Don't drink the water. Don't even open your mouth in the shower. Brush your teeth with bottled water. Above all, don't buy food from street vendors. Eat in established restaurants and ask if the ice and water are purified.
Late night taxi riding should be avoided. There are many taxi stands (sitios) that are much safer as they register arrivals and departures. Flagging down a taxi after 11 o'clock at night (especially in the dowtown section) could mean a free three hour tour of the city with your eyes closed) and at the minimal cost of whatever the ATM says your limit is (three hours because they will wait until after midnight to take out more money). Be careful. use basic safety tips such as no flashy jewelry and walking in the middle of streets so you can see in doorways, etc. and you should be OK. It isn't as bad as many people seem to think, but it is pretty bad on the whole.
O80 is the number to dial in the event of an emergency.
The same principles apply in Mexico as in any other country.