I know it's rough duty, but I'm sitting on the beach along the Baja Peninsula in Mexico testing wireless, and various flavors of tequila, available south of the border. Use of 911 as the official number for all emergencies in Mexico was to have officially started October 3rd of 2016. Rollout was to have occurred in 16 states across Mexico, not including Mexico City, and it may have. From my perspective here in one of the "official" states, it's not an obvious change. There are still signs along roads, posted at police and highway facilities, even on ambulances, showing the previous emergency numbers, 074, 066, etc. The multiplicity of numbers is part of reason for the change.
There have been memes and articles criticizing the change, which was originally scheduled to start on the first of 2016. The 911 task force understands there are challenges to the implementation, not the least of which are the thousands of regular phone numbers that begin with "911..." which must be changed to reduce mis-directed calls.
Fortunately for the US tourist, the majority of US phones that roam in Mexico will automatically interpret a call to 911 as one looking for a local emergency number, and find it. It should give us some comfort, especially since the incentive to choose a single national emergency number came after a tragic event that ocurred in Mexico in 2014.