Speed Limit to Increase on some roads

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gzt

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#1852110 24-Aug-2017 20:29

Linuxluver: Above 80kph drag increases exponentially. So going from 100kph to 110 increases emissions and does so out of proportion to the increase in speed.

Until current wind speed and direction is added to those journey planner apps : ).

gzt

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#1852116 24-Aug-2017 20:40

Linuxluver: I can happily do 110kph in my electric car, but the next one is going to have Bioweapon Defense Mode to filter out all the diesel exhaust from my air intake. It's really bad out there some days...

It's a good point. Even in Auckland city coastal and windy it's a problem, but it's not limited to diesel.

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#1852117 24-Aug-2017 20:40

gzt:
Linuxluver: Above 80kph drag increases exponentially. So going from 100kph to 110 increases emissions and does so out of proportion to the increase in speed.

Until current wind speed and direction is added to those journey planner apps : ).

My car has a very accurate fuel consumption meter. Whatever speed I go, it returns a ball park of 8L / 100ks. Yes I've done "experiments". Just gives me 8.

It probably costs more carbon dioxide to keep accelerating from 60-80 and then braking to keep it at 80.

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#1852119 24-Aug-2017 20:43

Mind you I also had an SUV before this, and the fuel consumption between 50-110 kph is proportional. 50 =5, 110 = 11, but beyond that speed stays at 11.

*Tsk disclaimer - I am well travelled, so I could well be doing those speeds on the Autobahn ok

Aredwood

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#1852166 24-Aug-2017 23:11

Linuxluver:
Above 80kph drag increases exponentially. So going from 100kph to 110 increases emissions and does so out of proportion to the increase in speed.

But that's just CO2.

The above only applies when your power source has an easily predictable efficiency / output curve. (electric motors do - and EV normally have a single ratio gearbox linking the motor to the wheels)

But Ice cars have gearboxes with multiple gears, and if the gearbox is an Automatic gearbox - it will also have a torque converter. Which couples mechanical energy via a fluid coupling, the fluid coupling allows "slip" to happen at low speeds, full throttle, and it is what allows an automatic gearbox to apply power to the wheels while the car is stopped. The torque converter also has a lockup clutch, to give better fuel economy on open road cruising.

Petrol and diesel engines have a RPM range where they are the most efficient. On older designs, it is often quite a narrow range. (old diesel engines especially often have very narrow peak efficiency ranges) More modern engines with variable valve timing have a larger efficiency range. Typically a car manufacturer will choose gearbox ratios so when the gearbox is in top gear, and the car is travelling at the open road speedlimit in the country it was made for. The engine will be in it's peak RPM efficiency range.

Of course not every car will fit the above. As you get models with underpowered engines for the size of the car. Or models where a new engine has been released, but without an updated gearbox.

If you are constantly driving at less than the design speed, often the gearbox changes down alot more often, or the lockup clutch has to keep disengaging. Both mean the engine is doing more rotations compared to each wheel rotation. Or it is simply operating below it's peak efficiency band. So often the actual fuel savings are less than what a simple calculation will give.

Other considerations:

Longer journey times - meaning more risk of accidents due to microsleeping while driving. Work time restrictions on commercial drivers, will mean more truck drivers will be needed for the same amount of distance covered by trucks.

The poor standard of driver training. Lots of people on the roads got their licence when the practical test was driving around the block in a small country town, with an MOT officer. And they have not had another practical driving test in 40+ years. Even myself - I remember getting my full drivers licence at 16.5 years old (after doing learners, restricted, full tests). And thinking "I won't need to do another practical driving test for at least 50 years".

There should be practical retests every 10 years. But there will be lots of busybodies complain about that. As the general population thinks they are a good driver themselves. And they think that most other people are bad drivers. Only 1 of those things can be true.

PhantomNVD

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#1852167 25-Aug-2017 00:00

But TBH, having scored 100% on a class 1 test for my P endorsement 2 years ago that makes me the one who IS a ‘good driver’ and the rest of you the self deluded deniers?
/joke

Linuxluver

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#1852693 25-Aug-2017 15:43

MikeB4: Too many drivers read the 100k limit as 110-115k the same drivers will now see 110 as 120-125k and apply it to all roads and all conditions and also see as the minimum speed.

Depends on the enforcement.

On the "400" highways in Ontario, Canada, the posted speed limit is 100kph. The edge of the road is littered with "Safety through enforcement" signed as you drive along.

I've never seen it enforced. Most people do 120 (outside lane) and anything from 130kph on up in the inner-most lane. I've been driving along at 120kph and the OPP (Ontario Provincial Police) pass me at 130kph (and the rest)....no lights / siren.

My brother considers 130kph to be norm.....in winter....with snow (as long as they salt it).

Crazy......

The 401 at Weston Road in Toronto is supposedly the busiest stretch of motorway in the world (maybe no one has stats for China).

This photo gives you some idea of scale. I've driven this many times.....so many lanes and the 'boy racers' are weaving across it making it a dodgy drive even if you are careful.

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Linuxluver

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#1852913 26-Aug-2017 11:01

Batman:

gzt:
Linuxluver: Above 80kph drag increases exponentially. So going from 100kph to 110 increases emissions and does so out of proportion to the increase in speed.

Until current wind speed and direction is added to those journey planner apps : ).

My car has a very accurate fuel consumption meter. Whatever speed I go, it returns a ball park of 8L / 100ks. Yes I've done "experiments". Just gives me 8.

It probably costs more carbon dioxide to keep accelerating from 60-80 and then braking to keep it at 80.

That isn't supported by more rigorous research on the subject, I'm afraid. In particular, see "Figure 4: Impact of travelling speed on various pollutants (Euro 4 gasoline passenger cars, 1.4–2.0 litre engine capacity)"

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