It has grown from 17,904 to 18,423 between 2015 and Aug 2017 .
The World Openness Score is a measure that tracks the ability of the world 's population to travel visa - free .
World Openness Score has grown from 17,904 to 18,423 between 2015 and Aug 2017 .
The World Openness Score is a measure that tracks the ability of the world 's population to travel visa - free .