Self driving cars… no thanks.

[rareposter]

You actually don't need the rails, or the big engine... if the cars are all talking to each other correctly... then a convoy system is a pretty easy thing to do on motorways.

You can achieve the same effect for vastly less time, effort, money and resource by reducing the speed limit a bit (even if only temporarily) which allows vehicles to be closer.

Counter-intuitively, slower speeds maximises throughput of vehicles, mostly by allowing them to be closer and with less variance in speed.

Trying to have autonomous vehicles all talking to each other and the road network is a wildly expensive and complicated "solution" to a non-problem.

[Bmblbzzz]

A convoy of communicative cars allows closer following distances while maintaining safety, and increases efficiency by reducing aerodynamic losses, but not to the effect of linking them all up to each other and putting them on rails. As rareposter has said, but people seem to have missed the point.

[axel_knutt]

Reaction time is a bit of a woolly concept because it's so variable depending on circumstances. The national average reaction time from seeing a brake light to getting your foot from the accelerator to the brake is 300ms, but that's if you are anticipating the light, and are poised ready to react. The time it takes to wake up from a daydream and realise there's something to react to can be from a couple of seconds to the sky's the limit though, and people who've just woken up tend to brake hard first then decide how hard later.

Maintaining a safe distance behind the car in front is a feedback loop: the gap you see determines braking and braking determines the gap you see, but the problem with feedback loops is that if any time delays are large enough to be significant, negative feedback becomes underdamped and unstable, and the faster you're going the more significant any given delay will be.

https://www.youtube.com/watch?v=7wm-pZp_mi0
https://www.youtube.com/watch?v=Rryu85BtALM

I wonder how many people allow more space behind an autonomous car that has faster reactions than they do.

[[XAP]Bob]

Trying to have autonomous vehicles all talking to each other and the road network is a wildly expensive and complicated "solution" to a non-problem.

They don't need to talk to the road network, just to each other... and a simple mechanism would be millisecond blinking of tail lights... completely invisible to the humans, trivial to implement and for a computer to spot.

I wonder how many people allow more space behind an autonomous car that has faster reactions than they do.

Not really necessary - you're reacting to it stopping, or to vehicle a way ahead... neither of which are affected by the reaction of the vehicle in front of you (if it's autonomous it will also be monitoring how close you are).
But I do adjust my following distance according the performance of the vehicle ahead... if it's a sports car then I'll leave a bigger gap, because it's probably got better brakes than me.

[Stevek76]

You actually don't need the rails, or the big engine... if the cars are all talking to each other correctly... then a convoy system is a pretty easy thing to do on motorways.

Counter-intuitively, slower speeds maximises throughput of vehicles, mostly by allowing them to be closer and with less variance in speed.

Not really, the key determinant of capacity is the headway in time, not distance. Whilst the distance might go down with slower speeds, the time stays more or less the same (up until things get so slow that vehicle length becomes significant).

In multilane conditions, average speed enforced 50 limits can help increase reliability though not necessarily peak flow which actually drops a bit (though increased reliability means the avg capacity goes up a little)

Not that I disagree with the rest of your point, the idea of convoys of AVs simply isn't a practical concept in most situations. The limited locations it might be like motorway links between junctions generally aren't the bottlenecks.

Same logic fail as Musk's idea of endless tunnels to solve congestion - it's the getting to and from the tunnels that is the problem.

[Bmblbzzz]

AV convoys are usually spoken of as a means of saving energy not increasing capacity. But as already pointed out, in both aspects they are inferior to an actual train.

[[XAP]Bob]

AV convoys are usually spoken of as a means of saving energy not increasing capacity. But as already pointed out, in both aspects they are inferior to an actual train.

And a train is massively inferior if you want to get somewhere that isn't a train station...

[[XAP]Bob]

You actually don't need the rails, or the big engine... if the cars are all talking to each other correctly... then a convoy system is a pretty easy thing to do on motorways.

Counter-intuitively, slower speeds maximises throughput of vehicles, mostly by allowing them to be closer and with less variance in speed.

Not really, the key determinant of capacity is the headway in time, not distance. Whilst the distance might go down with slower speeds, the time stays more or less the same (up until things get so slow that vehicle length becomes significant).

In multilane conditions, average speed enforced 50 limits can help increase reliability though not necessarily peak flow which actually drops a bit (though increased reliability means the avg capacity goes up a little)

This - if you have a two second gap between vehicles then the change in speed only changes how long it takes a vehicle to pass a spot, and therefore the time between the 2 second gaps.
If you have heavy congestion what a slower limit does is a) stop traffic arriving further up the road and b) stop people accelerating and braking. That gives time for brake light congestion to ease and will increase the "real world" throughput as a result - it's still below the peak, but not much.

[Bmblbzzz]

AV convoys are usually spoken of as a means of saving energy not increasing capacity. But as already pointed out, in both aspects they are inferior to an actual train.

And a train is massively inferior if you want to get somewhere that isn't a train station...

Not if it has a decent cycle capacity!

[[XAP]Bob]

Yes it is... it just fundamentally is.
Remember that you're part of a very small minority would would consider a bike to be completely adequate transport for anything, with a train being a supplement when you need to travel very long distances.

For most people it's just not. How do you suggest I carry my wheelchair on a bike/train multi-modal journey?
I know I am also part of a minority in that particular use case, but plenty of people - particularly when making long journeys - have multiple people, and multiple people's worth of luggage.

Of course that even assumes a train goes anywhere remotely like the direction you want to travel.
A couple of jobs ago I'd have commuted by train, but ignoring the ~£80/day cost, which would likely have been 160/month...
Assuming a 15 minute cycle at either end of the line...
I'd have needed to leave home three hours after arriving back from work to do a normal day in the office.
If I pushed the day later (which I could have done) then I'd have left home at ~6:40 am, to get to the office for 10, worked till 6, and got home at 9:30 - that's just 9 hours at home, hopefully 8 of which would have been sleeping.

A convoy system on the A14 meanwhile would probably have reduced the hour's journey time, and certainly been less tiring.

[Bmblbzzz]

Sure, all those points about last mile journeys apply. But you wouldn't be in a convoy on those parts either.

[[XAP]Bob]

No, but you also wouldn't be changing mode to split from the convoy and do that last mile.
You get the undoubted convenience of getting to your destination, rather than landing in "London Luton", and the ease and efficiency of spending most of that journey in a convoy.

They're only appropriate for a motorway scenario anyway - so it's only the long distance bits where the benefits are available/realisable. Designate the rightmost lane as a convoy and they don't even need to do anything about junctions.

[Bmblbzzz]

Once again, 500 people travelling from London to Edinburgh in a train is far more efficient (in terms of energy, resources and probably time too) than in 250 cars.

[axel_knutt]

Not really, the key determinant of capacity is the headway in time, not distance. Whilst the distance might go down with slower speeds, the time stays more or less the same (up until things get so slow that vehicle length becomes significant).

Stopping distance is proportional to the square of the speed so if you halve the speed the spacing can be a lot less than than half, so the capacity increases. The real bugbear is reaction time though, and the effect of that is less significant at slower speeds.

AV convoys are usually spoken of as a means of saving energy not increasing capacity. But as already pointed out, in both aspects they are inferior to an actual train.

And a train is massively inferior if you want to get somewhere that isn't a train station...

Or don't want to wait for the next train. My commute home:

17.01 Finish work
17.10 Train leaves
17.13 Arrive at station
17.56 Next train leaves
18.35 Train arrives
18.40 Arrive home

Or by car:

17.01 Finish work
17.02 Get in car
17.40 Arrive home

[[XAP]Bob]

Once again, 500 people travelling from London to Edinburgh in a train is far more efficient (in terms of energy, resources and probably time too) than in 250 cars.

Undoubtedly - but if you're not going from station to station it's not nearly as convenient, which was my point... You can have a good number of vehicles on the M1 joining and leaving a convoy all the way from London to Leeds with the efficiency gains *compared with solo driving*, as well as safety gains, whilst maintaining the convenience of actually going to where you want to go, rather than to a train station and then needing onward travel.