What regulations should a European driver know when driving in the US?

To me, these are the two most important things drivers visiting the US should know:

1. Different states have different laws. Knowing what the law is in California doesn't help if you get pulled over in one of the other 49 states.

2. Get your information from a trusted source. There's a lot of misinformation out there; the only way to know what the law really says is to get it directly from the state.

   For instance, here's what California's official Driver Handbook says about intersections (below emphasis mine):

   • At intersections without "STOP" or "YIELD" signs, slow down and be ready to stop. Yield to traffic and pedestrians already in the intersection or just entering the intersection. Also, yield to the vehicle or bicycle which arrives first, or to the vehicle or bicycle on your right if it reaches the intersection at the same time as you.
   • At "T" intersections without "STOP" or "YIELD" signs, yield to traffic and pedestrians on the through road. They have the right-of-way.
   • When you turn left, give the right-of-way to all vehicles approaching that are close enough to be dangerous. Also, look for motorcyclists, bicyclists, and pedestrians. When you turn right, be sure to check for pedestrians crossing the street and bicyclists coming up behind you on the right. On divided, highways or highways with several lanes, watch for vehicles coming in any lane you cross. Turn either left or right only when it is safe.
   • When there are "STOP" signs at all corners, stop first then follow the above rules.
   • If you have parked off the road or are leaving a parking lot, etc., yield to traffic before reentering the road.

This isn't a regulation, but it could be very important nonetheless. If stopped by the police, do not have the reflex of reaching into the glove box for the car's papers. This could be interpreted as reaching for a gun, and you risk getting shot preemptively. Once you've stopped your car, remain in your seat, and keep your hands well in evidence (on the steering wheel).

See also more recommendations by a US police officer.

The following apply to California versus Sweden, at least:

• In California, at uncontrolled intersections (without traffic signs or signals) you yield in theory "to the vehicle or bicycle on your right if it reaches the intersection at the same time as you" (Priority to the right). In practice, though, most intersections are controlled by stop signs or traffic lights, with uncontrolled intersections only in very undeveloped areas with little traffic, making this mostly a non-issue. In Sweden, the same "Priority to the right" [högerregeln] system applies, but the big difference is that many intersections are not controlled, and there are additional signs (priority road, end of priority road, intersection shape and priority) and rules (utfartsregeln, svängningsregeln, etc) to negate the priority to the right or otherwise modify the right of way, making it critical to understand and keep in mind "Priority to the right" all the time.

• In California, you're advised to "drive in the lane with the smoothest flow of traffic", though in general you're expected to keep to the right (if two lanes) or choose the middle lane (if three lanes). In Sweden, that's true but there's an important exception: if there are two lanes in the same direction and the speed limit is 70 km/h or lower, you are free to drive in any lane and may even pass to the right.

• In California, there is no guideline for when another driver is passing you. In Sweden, if another driver is trying to pass you, even if the pass is wrong or dangerous, you are obligated to facilitate the pass by moving to the right. You may specifically even drive on the shoulder if it is reasonable.

• In California, when entering a freeway, "freeway traffic has the right-of-way". In Sweden, if a specific on-ramp exists, neither side has right-of-way. Both sides are obligated to show mutual consideration when merging.

• In California, you must wear seat belts at all times. In Sweden, seat belts are specifically not required when backing the vehicle or when driving in a parking lot, gas station, etc.

• In California, you may turn right on red ("you may turn right if there is no sign to prohibit the turn"). In Sweden, there is no such provision, though there is an obscure case where this is possible. If there is a "Stopp vid signal" sign [Stop for the signal] and the traffic light is located after the intersection, the light only applies to you if you are crossing the intersection. If you're turning right, the light does not apply and you may effectively turn right on red. (This is not common.)

In most of (mainland) Europe there's a priority for traffic coming from the right (on an intersection without traffic lights), and you don't necessarily have to stop when you see there's no other traffic.

So one of the things I think a lot of Europeans will have trouble with at first when driving in America is the fact that on an intersection without traffic lights, there's a system of first come first serve, and you ALWAYS HAVE TO STOP (apparently - see the comments - you only have to stop when there is a stop sign). There are a lot of intersections in America which have a stop sign on all four directions, then the first come, first serve system is applied. That's something which doesn't exist in Europe because you simply don't have intersections with stop signs in all directions (mostly only 2 of them and the main road has priority). At least I don't know of any.

I say at first, because I think it's a very good (if not better) system once you get used to it. But that might also have something to do with the fact that most intersections in America are much larger than they are in Europe, so it's usually much easier to see all the traffic coming from the different directions.

In the United States, a traffic light showing a green left arrow indicates a "protected left turn": oncoming traffic has a red signal, as do conflicting pedestrian crossings. Here is an example, taken from an article by the Federal Highway Administration:

In Europe, a directional arrow signal does not mean that opposing traffic has a red signal. It just means that you can proceed in the direction indicated by the arrow, but still must yield the right of way as usual.

I was once in Italy, turning left at a 4-way intersection where traffic had to turn left or right because the street directly ahead was a one-way street in the wrong direction. To indicate this, the signal showed a left green arrow and a right green arrow. Unfortunately, I irritated an oncoming driver by cutting in front of him. He also irritated me because I thought he was running a red light. It was only a few moments later that I realized that I had been in the wrong, and why.

I can imagine that a European driver in the United States, facing a green left arrow, with cars approaching the intersection from the other direction, would probably wait before turning left, at least until it was clear that those cars were stopping. Any American drivers behind that European would most likely be irritated by that behavior.

Another difference with traffic signals: Most European signals are designed so it is not possible to see the signal for cross traffic, but this is possible with most American signals.

American intersections typically have at least one signal head on the far side of the intersection. Among other advantages, this allows the driver stopped at the stop line to see when the signal changes, because the signal is several meters ahead rather than directly overhead. See, for example, this image:

A consequence of this is that when you are waiting at a red light, you can see the signal for the cross traffic at a very oblique angle. You can usually see which lamp is illuminated by looking for the light reflecting from the sun shades. When the light for the cross traffic turns yellow, you can prepare to start driving.

European signals are generally placed only at the near side of the intersection. Some consequences of this difference:

• In Europe, when you stop your car at the stop line, you can't see the main signal head (unless you have a convertible or perhaps a sun roof). Most signals have a small signal head mounted low on the pole, so the driver of the first car can see when to go. (If you are stopped at a signal that lacks this second, smaller light, and you are tall like me, you have a very difficult time seeing the signal.) An example:

• If you allow your car to stop a little bit past the stop line, you are completely out of luck because you cannot see the signal at all.
• Many European countries have a signal phase immediately preceding the green light, to warn drivers to prepare to start driving. A couple of seconds before the light turns green, the yellow light turns on, so the signal is showing both red and yellow. This is not necessary in the US, because drivers can see when the signals are changing. An example:

• In some US intersections, of course, the light doesn't turn green immediately after the cross traffic gets a red light, because, for example, there might be a protected left turn for opposing traffic, or the intersection might be unusually complicated. In such cases, there is often a sign saying "DELAYED GREEN" or "DELAYED GREEN WAIT" in black letters on a white background. This is supposed to tell drivers not to start driving when the cross signal turns red, and to wait patiently for their own light to turn green.

When I was a kid, I had no idea why one might have a "delayed green" -- I always thought, "of course it's delayed; if the green doesn't wait for the other light to turn red, then what's the point of having a traffic light?"

Finally, the delayed green signal can be an arrow indicating a protected left turn. Here is an example from Harrisonburg, Virginia (see the hburgnews.com article for a discussion):

(The five-lamp signal has, at the top, a circular red light, with yellow and green arrows on the left, and yellow and green circles on the right.)

In California (and possibly in some other states) there is one strange regulation that states that the driveway starts at the curb so be careful when parking. The laws are very strict and it cost me 110 dollars because my car was slightly overlaping with the curb (50cm!).

Corrected: Great question, but your presumption is partially incorrect: the US does have a not have a keep-your-lane system in many states, except where signs explicitly show, but not all. Many drivers do drive that way, but in fact you are supposed to behave in a similar (although not identical) way to how you do in Europe: stay to the right (or left) by default, moving over only to overtake..

This Wikipedia Page and this page summarise the laws state-by-state.