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Comment by datavirtue | Hacker News Reader

How is the fastest route not the least carbon footprint?

Because the fastest route doesn't have to be the shortest route. Where I live, it's often faster to drive in the opposite direction of my destination to get on a highway. It's generally slower (by a tiny amount) to take surface streets.

The only thing that matters for your car's carbon footprint is how much gas you use. So regardless of time, if you have more gas in the tank at the end of your trip, your carbon footprint is lower.

The faster you go, the more your fuel efficiency decreases. For the most part this is due to wind resistance, because there is a velocity^2 argument there(ie if you double your velocity, you quadruple your wind resistance). Your engine is also designed to operate more efficiently at certain speeds but that is less directly quantifiable than wind resistance.

So you might be able to get 40MPG while doing a steady 40 mph, but only get 20MPG when doing 80mph.

So if you need to go 40 miles, if you can do it at 80mph it will take you 30 minutes, and will cost you 2 gallons of fuel. If you do it at 40mph it will take you 1 hour and cost you 1 gallon of fuel. So in this contrived example the longer trip has a lower carbon footprint.

In the real world, there are rarely two parallel roads going exactly the same place but just with a different speed limit. Usually you need to drive out of your way to get onto a highway to make your trip go faster. On the other hand driving directly at a slower speed usually entails more starts/stops at traffic lights which eats into your fuel efficiency. Google is probably in a good place to answer the question of how fuel efficient is a given route because they have so much data about average speeds, accelerations/decelerations required/ etc.

Efficiency drops sharply at high speed.

For long distance driving speed is key. Highways can be faster even when they are indirect. This is what the OSMAnd "Fuel Efficient Way" routing is based on. It biases towards the shortest routes that may have lower speed limits. This is nice for road trips and bicycling, but can be quirky with city driving.

OK...so google just fixed maps then. I don't know anyone who wants to get on the highway if they don't have to.

Maybe it's the shortest total distance, but has a lot of stop lights along the way causing frequent total stops/starts

The reason for the enactment of the nationwide 55mph speed limit in 1974 wasn't safety, it was oil prices.

Yes, though like many other American reactions to the oil shock, this was pretty silly.

High petrol prices automatically discourage speeding. (Just as a carbon tax would.)

Of course, the US also had price controls on petrol. And the ensuing long queues and fights at the petrol station.

Their anarcho-capitalist utopian neighbour [1] to the north did not enact price controls for petrol, and subsequently did not see any queues or fights.

[1] Only half joking here: in many respects Canada is more what we'd call neoliberal today than the US. Compare also https://www.alt-m.org/2015/07/29/there-was-no-place-like-can...

It looks like part of the carbon footprint model is based on road inclines and traffic congestion.

For example, a super-fast route that's up an extremely-steep incline on the way may technically be faster from point A to B, but would be less carbon efficient than taking a slightly-longer route that is flatter.

Driving a Chevy Volt (with an 18 kWh battery) makes you very aware of the different fuel-costs of different routes -- I very often have a choice of driving "over" or "around" small mountains (~hundreds of feet of elevation change, not ~thousands), and you can easily see the battery drop as you go up, up, up and it doesn't always recover much going down, down, down.

Going up and down mountains should be mostly a wash, apart from these small losses.

What's really eating into your battery in a way you can't recover with regenerative breaking ever is wind resistance.

Routes that make you drive faster for longer cost more energy. (And given the way wind resistance works, for the same average speed, a variable speed is more expensive than a constant one.)

Optimal regenerative braking recovers like 2/3 of the energy lost, and suboptimal braking -- for instance, if you need to decelerate more rapidly because of a bend in the road -- is worse.

Every 100 feet of elevation change amounts to ~1% of the battery capacity of a Chevy Volt; a couple of small mountains, with lots of ups and downs, and it's easy to lose 5-10% of your range versus taking a more level route, 5% if you brake well, 10% if you brake poorly.

[Which is not to say that hitting the thruway doesn't also kill your range fast.]

Downshifting is the devil.

Idling downhill costs less gas but may lead you in the wrong direction for a short while, vs going over a hilly route where you must accelerate uphill more often.

As a simple, unimaginative example. I'm sure you can think of others if you put in the time.

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