It is from one city to the other which the people of these cities consider that to be travelling.
The distance is not a problem. The problem is in the reality of travelling. If you are in the other city and someone calls your house, no one would usually say that you are travelling to that other city. People usually consider it another neighborhood and not another city. In Saudi Arabia, people of Jeddah don’t consider going to Makkah to be traveling. That is why, we don’t shorten nor combine prayers.