One American idiosyncracy that comes as a shock to many travelers to the United States from abroad is how particular Americans are about their personal space. Due largely to the country's geographic size, most American cities are much less densely populated than those in other countries. Businesses and homes tend to be spread further apart. Thus, Americans are very accustomed to having a certain amount of personal space.
When speaking face-to-face with an American, it's important not to get too close. Americans find this aggressive, particularly when being addressed by a stranger, and are less likely to respond in a positive manner. This is not to say that Americans are not friendly. Quite the contrary, when approached as they are accustomed, the majority of Americans are more than willing travelers in need of assistance.
If you are traveling through the United States and find yourself in need of directions or have a question that prompts you to approach an American, smile, greet the individual, and stop at least a foot away to speak. Don't be intimidated if the individual you approach looks suspicious at first. Once you explain that you are traveling and need assistance, the individual will become more relaxed as long as you maintain a reasonable distance during the conversation.
On buses and trains, it's also important not to stand or sit in a way in which you are touching another passenger unless the vehicle is at capacity and you have no choice. Also, Even then, when a stranger makes contact, it's instinctual for most Americans to take a step in a direction in which they believe there to be more available space. If you happen to bump into an American, it's important to apologize. Apologizing after bumping into people in the United States is less about expressing regret for an action and more about acknowledging that you just crossed into their personal space.
The United States is by and large a friendly country to visit. Americans are typically courteous and helpful. Respecting one's personal space in the United States, however, is a subtle way of expressing respect the individual. So when addressing, standing, or sitting near Americans, be mindful of personal space.