Logical Reasoning

PT114 · S4 · Q14 Reducing speed limits neither saves lives

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Reducing speed limits neither saves lives nor protects the environment.

Conclusion

Reducing speed limits neither saves lives nor protects the environment.

Evidence (This is because)

The more slowly a car is driven, the more time it pends on the road spewing exhaust into the air and running the risk of colliding with other vehicles.

Evaluate

This is one for which we could definitely react with our common sense about the world. We know that driving faster is more dangerous and less eco-friendly.

Why is it more dangerous? The author thinks driving slower is more dangerous because you spend more time on the road (and any second spent on the road is time you could be getting into accidents).

Well, that's silly. If I'm in a traffic jam for an hour, trudging along at 2-5mph, I'll be on the highway longer than if there were no traffic and I could just zoom through in 10 mins at 70mph. But I run zero risk of dying in a car accident if I'm going 2-5mph, whereas I could easily die in an accident going 70 mph.

Risk of dying in a car accident is in part a function of how long you're driving (as the author is thinking) but it's also a function of how fast you're driving / how in control of the vehicle you are.

Why do we know that driving fast is worse for the environment? Well, the faster you go the more gas you're burning. Cars are optimized to go highway speed (55mph) so they get their best gas mileage there. If you go 70 / 80mph, you're burning gas at a less efficient rate.

So even though a slower trip means you're "spewing exhaust" for longer, it's not at the same rate as when you're driving faster. Getting 30 mpg going moderate speeds for 1 hr can spew less exhaust than getting 20 mpg going high speeds for 45 mins.

Goal

Ultimately, since this is Flaw, we need to think about how to describe these problems in a more abstract way.

The author focuses on one aspect of risk to life (how long you're on the road) while ignoring others (the higher risk of fatality in a high speed collision).

The author focuses on one aspect of environmental damage (how long you're driving) while ignoring other aspects (the rate of exhaust spewing, based on how efficiently / inefficiently you're running the engine).

14.

The argument's reasoning is flawed because the argument

neglects the fact that some
Not an Objection
Not only would the claim "some (at least one) motorists completely ignore speed limits" be far too weak to be a good objection, it's also a strengthener in this case. The author is saying that reducing speed limits won't have these two effects. If people don't even pay attention to the speed limit, then reducing it probably won't have an effect.
3% picked this
ignores the possibility of benefits
Out of Scope
We can only hurt the argument by countering its conclusion, which is specifically about saving lives and protecting the environment. So any objection we raise needs to help us argue that reducing the speed limit could save lives or could protect the environment.
14% picked this
fails to consider that if
Not an Objection
When a Flaw answer choices starts with fails to consider / overlooks the possibility we can ask ourselves this, "(assuming the author really did fail to address this issue), would this idea weaken?" Would it hurt the author's argument if we said, "reducing speed limits would increase the number of cars on the road"? No that would seemingly strengthen. The author would say that "more cars = more chances for someone to get into an accident and more things spewing exhaust into the air".
5% picked this
Correct
presumes, without providing justification, that
Why this is right
When a Flaw answer choice begins with presumes / takes for granted / fails to establish we can ask ourselves, "Did the author need to assume this?" The author seems to think, "if car X spends more time spewing exhaust than car Y does, then car X is worse for the environment". So, yes, the author is assuming that time spent on the road is the #1 (if not the only) thing we need to consider. If we negate this assumption, would it become an objection? Negation: total emissions from a car for a given trip are primarily determined by something other than the amount of time the trip takes. Yes, that would be an objection. That would be like our objection that when you go 70 mph, you're working your engine much harder than when it's at 55 mph, and so you get worse fuel economy and potentially spew more total exhaust. If we were thinking, "wait this only weakens part of the conclusion; it doesn't say anything about the saving-lives part", you don't have to weaken both parts of a 2-part claim to weaken the argument. If I conclude, "Thus Sally will be at the party and she will eat that strawberry cake", I can hurt the argument by arguing that Sally won't be at the party, by saying she won't eat the strawberry cake, or both.
Skill tested: Flaw · how this choice captures the argument's function is the move to repeat next time.
59% picked this
presumes, without providing justification, that
Too Strong
Again, because of the presumes opening, we can ask ourselves whether the author needed to assume that "drivers run a significant risk of collision only if they spend a lot of time on the road". Hmm, that's really strong. Did the author really need to assume that "if you don't spend a lot of time on the road, you never run a significant risk of collision"? No, that's way too extreme. Our author might believe that an 80 year old driver with cataracts and worn out brakes runs a significant risk of collision, even if he only drives to Trader Joe's once every two weeks. Our author was assuming that "time spent on the road is a more important risk factor for fatal collision than how fast the driver is going", but nothing as black and white as this answer.
18% picked this

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