So if I'm driving along on a Motorway, at less than the local speed limit and, for some reason, a vehicle on the opposing carriageway comes over the central barrier and crashes into my car, I've not been involved in an accident?
Nope, that is why they are called Road Traffic Incidents these days.
You have changed the scenario a little here, in that there are now multiple humans and deadly machines involved. You may not be the one failing to recognise/flaunt appropriate risks but someone was. In this case the vehicle on the other carriageway was probably one or more of the following a) travelling too fast (it should take a lot to break a modern barrier), b) mechanically unsound, c) the driver was unfit to drive. There will alway be edge cases where it is determined that the likelyhood of risk was so low that the individual wouldn't normally consider them, or was able to take appropriate mitigations; for instance a falling meteor may have knocked them off course; but the default position is to say at least one person made a mistake. Investigations and conclusions later by HSE/Police/Coroner may determine that the risk was unlikely to be seen and mitigated by a cautious person and so it is an unforseeble accident.
You mention 'less than local speed limit' which is a good discussion point to think about risk and mitigations (and is no way a knock at you as I don't know your driving). In most cases the local speed limits are set at the national level (30/60/90). In some cases the controlling road authority has made some adjustment due to an increased perceived risk (e.g. 50 on a motorway with tighter than normal bends); but generally the driver is expected to drive at the appropriate speed for the road conditions and likelihood of risk. This is generally taken as the vehicle being in good working order and being able to stop in the clear distance that can be seen.
When we start to factor in other road users and possible risks should we be travelling slower?
- There is a pedestrian on a narrow road, probably yes.
- There is a falling rock signs and it's been raining recently; again maybe yes.
- The HGV on the other carriageway has a strap loose; maybe not.
- There's a cyclist on the other carriageway; probably not.
- There was meteor shower predicted for the day; I'm not going to slow.
There are static risks we assess and review occasionally (roads are more dangerous than house floors), but we should be doing a dynamic risk assessment every time we do something that may change what we do. Often this is sub-concious (floors are safe if there are no toys scattered on it, proceed as normal), but when the static risk assessment is higher we should be performing this dynamic risk assessement more consciously (continuously reviewing road risks and regularly adjusting speed, rather than sticking at 60 on single carriageway because it isn't raining).
Sorry, started as reply to Limey Lurker, but I went off on a tangent.