WHEN TO TAKE A TAXI
If you are traveling to Cancun Airport from anywhere in Cancun or up to Playa del Carmen you can take a taxi to the airport. You can take the bus from Playa del Carmen/Tulum to Cancun then take a taxi to the airport. The cost from the bus station to the airport is 60 pesos. The cost from Playa del Carmen is 100 pesos.
Playa del Carmen
You can walk everywhere in Playa and do not need a taxi unless you are in hurry or carrying a lot of stuff.
As big as Merida is if you stay in a hotel downtown then you are within walking distance to everything you may want to see. You can expect long walks and some of the streets are blistering hot on a sunny day. It is a rather long walk to the zoo so you may want to walk there and take a taxi back to your hotel. If you wish to visit Dzibilchaltun/Chichen Itza you need to take a bus or taxi.
Other than taking a taxi from the bus station or airport to a hotel downtown then you do not need a taxi unless you decide to visit the museum which is located 4 km outside of town or Edzna which is located 60 km away.
If you stay near the beach and wish to travel downtown or vice-versa then you will need to take a taxi or a collectevo [bus]. If you stay downtown and do not need to visit the beach then you only need a taxi to get to the bus station or airport. You need to take a bus or taxi to visit any ruin sites around Chetumal.
The ADO bus station is located within walking distance from town square. Take a right as you walk out the front door. You are within walking distance from everything and a short taxi ride [30 pesos] to the ruins, 6 pesos by collectivo [bus].
If you stay downtown you can walk to any destination however you need to take a taxi or bus to visit Chichen Itza.