After they were come to Mysia, they assayed to go into Bithynia: but the Spirit suffered them not.
Note 4 at Ac 16:7: Mysia was a province in the extreme northwest of Asia Minor. Troas (Ac 16:8) and Assos (Ac 20:13) were two of its cities that Paul passed through. In Re 1:11 and 2:12-17, the Apostle John mentioned the church at Pergamos (also known as Pergamon or Pergamum), also a city of Mysia.
Note 5 at Ac 16:7: Bithynia was a province in the northwestern portion of Asia Minor, just east of Mysia (see note 4 at this verse). It was bordered on the north by the Black Sea.
Note 6 at Ac 16:7: Some people have the mistaken idea that we should not go anywhere or do anything without the Lord giving us specific directions. That certainly was not the way that Paul was operating. Paul and his companions were just systematically taking the Gospel to everyone without the Lord telling them to "go here" or "go there." They were going everywhere (Mt 28:19-20) at their own discretion. Otherwise, the Holy Ghost would have been contradicting Himself.
This does not mean that Paul was insensitive to the leading of the Lord. When he received specific direction through this vision (Ac 16:9), he followed it gladly, but he was not waiting until the Lord gave him a specific destination. He was following the Lord's general command to all believers to go into all the world (Mt 28:19-20) and the Lord's special instructions to him to go to the Gentiles (Ac 9:15, see note 7 at Ac 9:17).
This is a very clear example that we should follow today. We as believers should be listening to the Lord for any special directions that He might choose to give us, but we should not be waiting on Him before we go witness to the world that the Lord has given us. Failure to understand this has caused many sincere people to do nothing in the false name of obedience. Instead of presuming that there is always a red light and waiting on the Lord for the "go ahead" of a green light, we should presume that the Lord has given us the green light to go into all the world, and then watch for a red light.