There is no fixed set of fault determination rules in the UK: after a road traffic accident, fault is decided by the insurers, or by a court if they can’t agree, on the balance of probabilities, using the Highway Code to judge how each driver should have behaved, together with the damage, witnesses, photos and dashcam footage, and where both drivers contributed, liability is split, for example 50/50 or 70/30. If the accident wasn’t your fault, National Accident Support Line handles the claim against the other driver’s insurer, and its in-house recovery team argues disputed liability on the evidence.
If you’ve searched for “fault determination rules”, you may have found pages about Canada. Ontario has a regulation with that name that sets out who is at fault in each type of crash. Britain doesn’t. Here, fault means negligence: which driver failed to drive with reasonable care, and by how much. In practice, though, most crashes fall into a handful of familiar patterns, and insurers settle them in familiar ways. This guide covers those patterns, what decides the disputed ones, and what to do if the other driver’s insurer says it was your fault when it wasn’t.
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How fault is decided after a car accident in the UK
Each driver tells their own insurer what happened. The two insurers then compare accounts and evidence and either agree who was to blame, agree to split it, or dispute it. Very few cases reach a court; when they do, a judge decides on the balance of probabilities, which means what is more likely than not to have happened.
The Highway Code sits behind all of it. Many of its rules are advice rather than law, but under section 38(7) of the Road Traffic Act 1988 a breach of any rule can be relied on to establish liability. So when an insurer says a driver should have given way, or should have left a bigger gap, it is usually pointing at a specific rule. For why those rules exist at all, see the importance of traffic laws.
The evidence that moves a disputed case, roughly in order of weight: dashcam or CCTV footage, independent witnesses, photos of the vehicles where they came to rest, where the damage is on each vehicle, and anything the police recorded. What was said at the roadside counts for less than people think. A roadside apology isn’t a formal admission, and accounts can differ once both drivers report the accident, which is why the evidence matters more than the apology.
Common accidents and who is usually at fault
These are the usual starting points. Each one can move if the evidence shows the other driver did something wrong too.
| What happened | Who is usually at fault | Highway Code |
|---|---|---|
| A car drove into the back of yours | The driver behind, who should leave enough distance to stop. It can shift if the car in front reversed into them or braked hard for no reason | Rule 126 |
| A car pulled out of a side road into you | The driver pulling out, who must give way. It can be shared if the main-road driver was speeding or overtaking at the junction | Rules 170-172 |
| A collision on a roundabout | The driver who didn’t give way to traffic from the right, or who changed lanes or exits without checking | Rules 184-187 |
| A car changed lanes into you | The driver who moved across | Rule 133 |
| You overtook a car that turned right | Often shared: you shouldn’t overtake at a junction, and the turning driver must check mirrors before turning | Rules 167 and 180 |
| A car reversed into you | The reversing driver. If both of you were reversing, as often happens in car parks, expect a split | Rules 200-203 |
| A car door was opened into your path | The person who opened the door | Rule 239 |
| You were hit while parked | The moving driver | |
| A driver turned across a cyclist or pedestrian | Usually the driver. Since January 2022 the Code places most responsibility on those who can do the greatest harm | Rules H1-H3 |
The full rules are on the Highway Code on GOV.UK.
Split liability, 50/50 and knock for knock
When both drivers were partly to blame, or nobody can prove what happened, liability is split. A 50/50 split is common where there are two different stories and no witnesses or footage. Other splits, such as 70/30 or 80/20, reflect who was more at fault. Under the Law Reform (Contributory Negligence) Act 1945, being partly to blame doesn’t stop a claim; it reduces what you recover by your share. If you were injured and not wearing a seatbelt, an injury claim can be reduced for that too.
Two things catch people out. First, a split usually counts as a fault claim on your insurance record, because your insurer can’t recover all of its costs. Second, “knock for knock” is not the same as 50/50. Knock for knock means the insurers don’t decide fault at all and each simply pays its own customer’s claim. People on forums often use the two terms as if they mean the same thing; they don’t.
When your insurer accepts fault without asking you
Most motor policies allow your insurer to decide how to settle a claim, including accepting liability or agreeing a 50/50 split, without your agreement. The Financial Ombudsman Service has said it doesn’t investigate accidents to decide who was responsible; it looks at whether the insurer considered the evidence fairly before making its decision. So if you disagree with how your insurer is handling liability, the time to say so is before it settles: send it your footage, photos and witness details in writing.
A claim also stays open until the insurer recovers its costs, and an open claim can affect your no-claims discount at renewal. If you’ve been found not at fault by the time you renew, the Ombudsman’s position is that your discount shouldn’t be affected, whether or not the claim has been closed.
What to do at the scene so fault can be proved
Stop, and give your name, address and registration to anyone who has reasonable grounds to ask. If you couldn’t exchange details, or someone was injured and you didn’t show your insurance certificate at the scene, you must report it to the police as soon as reasonably practicable and in any case within 24 hours. Then collect what will decide the claim: photos of both vehicles before they’re moved, the road layout, signs and road markings; the other driver’s details and insurer; names and numbers of witnesses; and your dashcam footage, saved before it records over. If the other driver drove off or gave false details, a hit and run has its own claims route through the Motor Insurers’ Bureau.
If the other driver’s insurer says it was your fault
This is the point where many people give up, or agree a split with their own insurer. You don’t have to. If the accident wasn’t your fault, National Accident Support Line handles the claim against the other driver’s insurer directly, so the claim doesn’t go through your own policy, though you should still let your insurer know the accident happened. We arrange a like-for-like replacement vehicle through credit hire and your repairs, and if the other side disputes liability, our in-house recovery team argues it with their insurer on the evidence. We take on disputed claims that many companies won’t. For everything else in the days after, from the phone calls to the paperwork, see what to expect once the accident wasn’t your fault.
If the evidence shows it was your fault, or that the blame will be shared, we tell you that on the call and point you back to your own insurer. If you were hurt, our page on injury claims after a road traffic accident explains how we refer those, and how a non-fault claim for your car works covers the vehicle side step by step.
Frequently asked questions
Who is usually at fault if a car hits the back of you?
The driver behind, in almost every case. Rule 126 of the Highway Code says drivers should leave enough distance to stop. It can change if you reversed into them or braked hard deliberately to cause a crash, but braking for a hazard is something every driver is allowed to do.
How do insurers decide who is at fault?
They compare both drivers’ accounts with the evidence: the damage, photos, witnesses, dashcam or CCTV and any police record, measured against the Highway Code. If they agree, the claim settles on that basis. If they can’t, it can go to court.
Is the driver pulling out of a junction always at fault?
Usually, but not always. The emerging driver must give way, but a main-road driver who was speeding, or overtaking at the junction, can share the blame.
Does a 50/50 split count as a fault claim?
Usually, yes. Your insurer can’t recover all of its costs, so the claim is normally recorded as a fault claim and your no-claims discount can be affected.
Can my insurer accept fault without asking me?
Most policies allow it. If you have evidence, get it to your insurer in writing before it settles. The Financial Ombudsman Service can consider whether the insurer weighed the evidence fairly, though it won’t decide who caused the accident.
What if the other driver admitted fault and then changed their story?
A roadside admission helps, but the claim turns on the evidence, so footage, photos and witnesses matter more than what was said.
Who is at fault if two cars reverse into each other in a car park?
Usually both. Where both drivers were reversing, the claim is often settled 50/50 unless one car had clearly stopped.
What if the accident wasn’t my fault but the other insurer disputes it?
Send us what you have. If the evidence supports you, we arrange your replacement vehicle and repairs and our recovery team argues liability with the other insurer directly.
No excess to pay if the accident wasn’t your fault. Fill in our form and we’ll call you back within 2 minutes.