United Kingdom

Basically 3 points to keep in mind.

1. Were you involved?

2. Do you have useful and relevant information (e.g. a dash cam that captured what happened)?

3. Can you provide additional aid?

The first one is obvious, if you're involved, you stop and at least provide contact details.

The 2nd is harder, but not too hard. If it happened right in front of you, pull over somewhere safe, if possible. If you can't, contact the non-emergency police line afterwards. They will be able to advise.

The last is the complex one. If you're the first on the scene, any aid is better than one. If you're not, then judge whether you can provide more help. E.g. 1 person trying to help multiple victims, or an elderly individual, who might not be physically capable of rendering aid.

A side note is providing protection. I drove a larger vehicle. I recently witnessed a motorway accident (driver fell asleep at the wheel). A depressing number of drivers were dangerous idiots. Several cars drove between stopped vehicles to get past. Some got very close to running over rescuers. I ended up using my van to block off the immediate area, and force traffic well clear.

Basically, keep yourself safe first, keep others safe second, render aid third. If you can't do any of those, either wait patiently, or maintain the traffic flow as best you can.

Former non uk firefighter here:

Rule number one is don't worsen or add to an existing accident. If you can't be absolutely sure traffic has safely stopped and is blocked, don't get out. Not only can someone get hurt, but if there is another accident it can worsen the access to the first, potentially more serious event. Even then car wrecks themselves are dangerous. If you are capable of providing aid, be sure not to injure yourself in the process.

Calling 911 as soon as possible is best (safely). You could drive past and pull over, well ahead.

Ultimately use these notes as guidelines. You will have to use your judgement, and may one-day decide the potential outweighes the risks, and you'll only have you to argue with in that moment

Roll your window down and yell "oi, you can't park there" while you pass.

I think you have to use your best judgement.

If there was a multicar crash and you were early to the scene, you could pull around them and stop in front and go to check in the occupants but you are in danger if the traffic hasn't slowed because someone else could pile into it.

In cases like yours, it's often best to keep the flow of traffic moving and you did the right thing (including hazards). If you have a passenger, they should report it to the police so they can get a car out asap. You can feel a little... impotent doing this but often it's the best thing.

Rules 282 and 283 of the Highway Code seem the key ones.