I've been to all of the continents on this planet other than the really cold one that only researchers go to way down south. One thing I have noticed in my travels is that the number one ripoff that exists all over the world is ripping people off for taxis. Sometimes you won't even realize that you have been ripped off until much later because perhaps you come from a part of the world where taxis just are expensive. I know this is the case in most western countries but don't let this ignorance have you parted with your cash. There are some easy ways to get around this.
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For starters, the first thing you need to do when you are arriving in a new area is to do just a cursory glance at some online forums to find out how much a taxi is meant to cost. You really should abandon any hope that you are going to score the same price as locals unless you are very stern about it. For the most part I expect to pay more than a local and I accept this provided it isn't something crazy like 4-5 times the price. Once you are armed with the information about how much the going rate is for a taxi you are in a much better position to bargain.
Next off is to NEVER get into a taxi until you have already negotiated a price. It is quite common for taxi drivers to take off before you have even said where you are going and then they kind of have you trapped. In most places the drivers will be standing near their cars and you can negotiate beforehand. It is probably a good idea to have small notes because there have been many instances I have found myself in where they claimed to not have change when I am relatively certain that they do.
Stand with the door open, or open the front door to talk to the driver before sitting down. Once a price is negotiated, if you can pay up front do so.
You need to keep in mind that at airports, most taxis will have some sort of "taxi rank" surcharge that they have to pay and of course you do not know how much this is. If they claim it is something huge they are almost certainly lying. Again, this is something that you can find out with a tiny bit of research.
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You may think that you will be "protected" by using an app like Uber, Lyft, or Grab but this is not always the case. In many parts of the world that I have personally been to, these apps have certain zones that are already dictated by the app to be "rippoff zones" and these areas are conveniently at places where only tourists end up. A good way to check to see if you are in one of these ripoff zones is to check to see a place a few blocks away as a potential pick-up point and see if the price drops a great deal. It has been my experience that it normally will. Unless you are physically incapable of walking a few blocks, I think it is worth it.
In one particularly bad situation where I was a bit upset with Grab I was at an airport in Laos and the Grab price was something I thought was pretty crazy, but if I simply moved my pickup point about 80 meters down the same street I was standing on, the price was 1/5 what it was from the international terminal. I can only imagine that this is part of the taxi union or mafia negotiation process that Uber and Grab must go through in order to be allowed to operate in these cities.
Just play around with it a little bit. I have seen massive shifts in the price based on just a block or so in dozens of cities around the world. This is not Grab or Uber trying to rip you off, I would imagine that their hands are tied.
Also, if you ever find yourself in a situation where a taxi driver starts to get aggressive with you, just walk away. There are so many taxi drivers in any given city in the world that you don't need to subject yourself to that sort of abuse. They aren't going to assault you in such a public setting. Once you learn the ropes of taxi strategy, it is actually pretty easy to not get ripped off. You will still likely be paying 20% more than locals do but if you want to save that extra amount all you have to do is become fluent in their culture and language.