If the slow driver is fully capable of driving a normal speed and makes the choice to drive well below the speed limit, they are making a choice that directly i

If the slow driver is fully capable of driving a normal speed and makes the choice to drive well below the speed limit, they are making a choice that directly increases the probability of an accident (regardless of if they are involved in the actual accident or not). If the slow car has an issue that makes it so that they cannot go any faster they should be putting their hazards on and slowing down, in which case it would be the drivers who are not paying attention's fault. Really in both cases the drivers not paying attention share some of the blame, but in the first case the slow driver is also being irresponsible.