I had heard (confirmed by this discussion and this one) that one may not do dangerous things during the Nine Days including "travel." Based on this: What types of travel are included? Plane? Car? Boat? Train? Walking? Bike? Hoverboard? (Etc. etc.) Does it matter how long the trip is? That is, what is the level of danger required to prohibit traveling?
Lich'orah, anything that you would need to say T'fillas HaDerech for.
We're looking for long answers that provide some explanation and context. Don't just give a one-line answer; explain why your answer is right, ideally with citations. Answers that don't include explanations may be removed.