Thing is , I don't think anyone on this forum witnessed the incident , so none of us know what really happened .
Absent any strong evidence either way , it appears that HRH emerged from a minor road into the path of a vehicle already traveling on the major road - that alone places liability on him , unless he can show good reason why it ought to be otherwise .
Notwithstanding his age , we are all human , and we all make mistakes , greater or lesser , when we go out in our cars ; most of the time we might inconvenience someone and a horn gets sounded or lights may be flashed , but there are no serious consequences , and those of us with the better attitudes might think 'that was close , and it was my fault , I'll be more careful in future' .
Sometimes , even those of us with good driving records may make an error which results in a collision , if we are lucky just a minor prang and a bit of damage which can be repaired ; if we are less lucky then injury or worse may occur .
It is worth bearing in mind that the outcome is not always proportionate to the degree of the error : we might make what seems like a very minor mistake that still has a catastrophic outcome ( pull out in front of an overtaking car that you didn't see ; said car does not hit you but goes onto the opposite side and has a head on collision or knocks someone down ) ; or someone may do something mind numbingly stupid ( think overtaking round a blind bend , or passing a bus stopped at a pedestrian crossing as examples ) and escape without consequence . That is why the courts have very clear guidelines to assess culpability .
However , there must be millions of minor incidents on the roads every day - while in almost every case charges could be brought , in most cases if the parties are of good character and driving history then this does not happen , and normally the cost of damage or minor injuries are dealt with by insurance , that then being the end of the matter .
There has been much tittle tattle in the media regarding the Duke's driving history and his manner of driving - absent irrefutable factual evidence this is just hearsay and would have no bearing on either a court case or a civil claim . It is likely that he has a 'clean' driving licence , otherwise much would already have been made of it - whether or not his vehicle was insured through an external insurer or whether the Royal fleet is self insured is neither here nor there , any liabilities will be met without question .
The reporting about the non wearing of seat belts is another non issue in itself - he is only placing himself at risk ; the Royals don't use the NHS so any consequential cost to himself from injuries arising from this would be bourne by himself . If he wants to pay the fines ( if they ever were given ) then that is up to himself . There may be an argument in that his apparently disregarding one piece of legislation ( we don't know that he does not have a medical exemption , although the vehicle is of an age where fitment and use of seatbelt is mandated ) goes towards a cavalier attitude towards driving and the law in general ...
I find it mildly amusing to consider that if this matter ever went to court ( which I doubt that it will ) then the case would be HM vs. HRH . I rather think that HRH will already have been tried and convicted in camera by HM and will now be serving penance in the doghouse at HM's displeasure
She is known to hate anything which reflects badly on 'The Firm' , and no doubt he will be feeling her wrath .
I suppose that , given that prosecutions are enacted in HM's name , then if she so decided , she 'could' decline to prosecute her husband , and deal with the matter privately , perhaps by banning him from driving on HER highway :-D Of course this is most unlikely to happen .