Collision of cars and how liability is determined

The answer – and Allah is the One who grants success – is that the scholars of the madhhab have divided mistaken killing into two types:
1 – That the killing occurs through a direct act.
2 – That it occurs through a causing factor (sabab); such as when a man digs a pit in a public road, or in a public right of way, and a person falls into it and dies; or when he places a stone in a public road and a passer-by trips because of it and falls and dies, and the like.
The scholars of the madhhab said: the one who has transgressed by committing a causative act – like the two mentioned examples – is liable for what occurred because of his pit or his stone. He is liable for the blood-money (diyah) of the one who died in that manner, and the diyah is borne by his ʿāqilah. Likewise, he is liable for what is less than full diyah, such as wounds and loss of property.
They also said: the one who dug the pit, and the like, is not required to perform kaffārah, because he did not directly kill the one who fell into the pit. Rather, the one who fell is the one who killed himself by falling into the pit. The digger only dug the pit and did no other act; and kaffārah is only required of one who is a mistaken killer, and the digger is not a killer – the doer of the killing is the one who fell.
We shall now present examples and scenarios of many types of car accidents in light of what we have understood from the words of the scholars of the madhhab in this chapter. Success is from Allah:
First scenario:
If a defect occurs in the car – such as the explosion of one of its tyres, or a defect arises in the braking system, or the like – and this leads to the car overturning or colliding with another car or with something else, then in such an accident, if death occurs as a result, no kaffārah is required. This is because, by the occurrence of the defect, the car has gone out of the driver’s control.
The scholars of the madhhab have said something similar concerning a ship when it goes out of control.
Second scenario:
If two cars collide because one of the cars has violated the traffic regulations commonly recognized among drivers, then whatever killing occurs in the two cars is upon the transgressing driver, because he is the one who caused the accident. There is no kaffārah upon him, because he is a doer of a causative act (sabab).
Third scenario:
If a collision or overturning occurs because of dirt on the roadway, or water, or a stone, or the like – things that cause the car to skid, veer off, or its tyre to explode – then liability for what occurs of deaths and other losses lies upon the one who placed the stone, the speed-bump, the dirt, the water, and the like, and there is no kaffārah. This is because the killing occurred by a causative factor, and liability lies upon the one who committed the causative act – if there is such a person. If the cause is from Allah, exalted is He [i.e., no human actor], then liability is attached to the driver and to his ʿāqilah.
Fourth scenario:
If a person falls from the back of the car and dies, then: if his fall was due to a speed-bump or a stone in the road, liability lies upon the one who placed the bump or the stone in the road, if he was transgressing in placing it, and there is no kaffārah.
If the person fell because of an action of the driver – such as his pressing the brakes or the like – then there is no liability and no kaffārah, unless the driver intended to make the passenger fall; in that case, he is liable, but there is still no kaffārah.
The scholars of the madhhab have mentioned something similar regarding someone who fires a rifle and a child, for example, dies from the sound of the gunshot. They said that there is no liability upon him unless he intended to kill the child by firing his rifle.
The reason why the driver is not held liable is that he has not transgressed in committing the causative act; he is permitted to press the brakes when danger suddenly appears. The one riding on the back of the vehicle is the one negligent in protecting himself.
Fifth scenario:
That the accident is a mistaken one caused by both drivers. In that case, the two drivers are jointly liable for what occurred because of their mistake, and each one is liable according to the extent of his mistake. There is no kaffārah.
Sixth scenario:
That a person steps out into the car’s lane and the car hits him and he dies. If the owner of the car was driving at the normal speed, then there is no liability and no kaffārah, because he has not transgressed in the causative act.
If he has transgressed in speed, then liability is upon him but not kaffārah. The normal amount of speed differs from one type of road to another: there is a specific limit for city streets, and so on.
Perhaps the reason why the usual practice of judges is to hold drivers liable is because, in most cases, the cause of cars hitting people is excessive speed beyond what it should be.
Seventh scenario:
That a collision or overturning occurs because of fog or rain. In that case, reference is made to the traffic regulations commonly known among drivers. If it is known among them that driving in such weather and on such roads is prohibited because of its danger, then the driver is liable, but there is no kaffārah upon him. He is held liable because he transgressed in committing the causative act.
If the common practice is to drive in such weather, then there is no liability and no kaffārah, because the driver has not transgressed in the causative act.
Eighth scenario:
That a camel, donkey, or the like enters the roadway and the car overturns by striking it. In that case, there is no liability and no kaffārah.
If the driver swerves the car so as not to strike the camel, donkey, another car, a person, or the like, and as a result the car either overturns or collides with another car, then: If the car overturns, what appears to me is that there is no liability and no kaffārah, because he has not transgressed by swerving the car from its path. If, by swerving the car from its path, a collision with another car occurs, then if he swerved to the point that the car ended up in a lane that was not its own, he is liable for what occurred in both cars, and there is no kaffārah. We only held him liable because he left his usual lane into the lane of the other car; we did not hold him liable merely for the act of swerving.
Ninth scenario:
That the driver is suddenly surprised by a sharp bend and is unable to slow down, and he finds no way to be safe except by turning the car into that bend at high speed, which leads to its overturning. If he knew the speed limit on that road and exceeded it, then he is liable because he transgressed in the causative act, but there is no kaffārah.
If he did not know and did not suspect any speed limits or the presence of dangerous bends, and so he drove fast, then there is no liability because he did not transgress in speed, and there is no kaffārah.
Yes, Allah, exalted is He, has made kaffārah and diyah obligatory in the case of killing a believer by mistake. He, exalted is He, says: “And whoever kills a believer by mistake – then (his atonement is) the freeing of a believing slave and a compensation payment handed over to the family of the deceased…” [al-Nisāʾ:92]. The apparent sense of the verse is that for every believer killed, there is both a kaffārah and a diyah. Therefore, diyah and kaffārah multiply according to the number of persons killed.
There is no disagreement that diyah multiplies according to the number of slain persons, and kaffārah goes with diyah as its parallel. What has made both obligatory is mistaken killing; so there is no basis for differentiating between diyah and kaffārah.

Source: Min Thimār al-ʿIlm wa al-Ḥikmah vol.2