A large number of drivers routinely violate highway speed limits. Sinc

correct assumption
C. A driver seldom needs to exceed the speed limit to avoid an accident when none of the other drivers involved are violating the speed limit.

What does the argument state -

Installing devices --> Most accidents would be prevented.

A - irrelevant to the argument above. Whether the device can be installed by trained or untrained people has no bearing on its effectiveness.
B - not relevant. the argument talks about the effectiveness of those devices. The amount of experience they have is irrelevant.
C - correct answer. Let us assume a value for seldom - say 1%.
Only about 1% of the time does a driver need to exceed the speed limit to avoid an accident when others are not exceeding the speed limit. This basically says that you do not need to exceed the speed limit to avoid an accident when everyone else is going slow. Strengthens.
Negate this -
About 95% of the time a driver needs to exceed speed limit to avoid an accident when others are not exceeding the speed limit. -> Destroys the argument. Suggests that accidents cannot be avoided if you impose speed limits.
Also, notice that "when others are not exceeding the speed limit" is an important part of the assumption. This depicts a scenario in which "all cars" (read the argument again) have such devices installed. If some people do, then such a scenario does not describe the situation that the argument deals with. (Since all cars have such devices installed, none of them will exceed the speed limit).
D - Negate this option. Most drivers who exceed the speed limit do so intentionally. -> If they do so intentionally, it is high time that they be stopped. This negation clearly strengthens the argument. Incorrect.
E - "increasing fines" has no impact on the effectiveness of the strategy. The argument only talks about the effectiveness of the strategy to install devices in order to reduce accidents. We are NOT discussing the effectiveness of the strategy to impose higher fines on speeding drivers.

A large number of drivers routinely violate highway speed limits. Since driving at speeds that exceed posted limits is a significant factor in most accidents, installing devices in all cars that prevent those cars from traveling faster than the speed limit would prevent most accidents.

What if one car requires to increase speed beyond limits to save an accident?
Author assumes this won't be a case normally.
SO-
"A driver seldom needs to exceed the speed limit to avoid an accident when none of the other drivers involved are violating the speed limit."
Ans. C.

Which one of the following is an assumption on which the argument depends?

A. A person need not be a trained mechanic to install the device properly.
B. Most accidents are caused by inexperienced drivers.
C. A driver seldom needs to exceed the speed limit to avoid an accident when none of the other drivers involved are violating the speed limit.
D. Most drivers who exceed the speed limit do so unintentionally.
E. Even if the fines for speed-limit violations were increased, the number of such violations would still not be reduced.

Premise: There are too many drivers who violate the speed limit and this violation cause a lot of accidents and speed is a significant factor of these accidents.

Conclusion: Installing devices to prevent those cars from travelling faster will prevent accidents.

Using @egmat's Pre-thinking method:

Falsification statement:
In what scenario, installing such devices won't prevent accidents?
Given that: high speed is the major factor of these accidents.

These devices won't prevent accidents if some vehicles need to increase the speed of vehicle to prevent some accident.
Negating the above statement will give assumption.

These devices will prevent accidents if some vehicles don't need to increase the speed of vehicles to prevent some accident.

A. A person need not be a trained mechanic to install the device properly. - Totally out of context
B. Most accidents are caused by inexperienced drivers. - It actually weakens the conclusion
C. A driver seldom needs to exceed the speed limit to avoid an accident when none of the other drivers involved are violating the speed limit. - Aligns with our assumption
D. Most drivers who exceed the speed limit do so unintentionally. - It definitely strengthen the conclusion about why devices should be installed but this cannot be an assumption because we don't know if drivers do that intentionally or unintentionally
E. Even if the fines for speed-limit violations were increased, the number of such violations would still not be reduced. - It might strengthen the conclusion but there can be other ways as well to reduce the accidents (speed breakers etc.)

The assumption on which the argument depends is (C) A driver seldom needs to exceed the speed limit to avoid an accident when none of the other drivers involved are violating the speed limit.

The argument assumes that installing devices in all cars that prevent them from traveling faster than the speed limit would prevent most accidents. However, this assumption is based on the idea that most accidents are caused by drivers who are exceeding the speed limit. If it were the case that drivers needed to exceed the speed limit to avoid accidents caused by other drivers who are violating the speed limit, then installing devices that prevent cars from exceeding the speed limit might actually increase the number of accidents. Therefore, the assumption that a driver seldom needs to exceed the speed limit to avoid an accident when none of the other drivers involved are violating the speed limit is necessary for the argument to hold.

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