4 ms·

> [1]: https://bigbuild.vic.gov.au/projects/metro-tunnel/about/technology/high-capacity-signalling https://bigbuild.vic.gov.au/projects/metro-tunnel/about/tech.

> [1]: https://bigbuild.vic.gov.au/projects/metro-tunnel/about/technology/high-capacity-signalling https://bigbuild.vic.gov.au/projects/metro-tunnel/about/tech... This video was great thanks! My naive assumption was that as trains operate in a relatively controlled environment a headway much smaller than 120s would've been easy to achieve. Cars regularly drive sub one second from each other! Is it just about minimizing the chance of catastrophe or is there more to it?

alfiedotwtf 5y ago
Even though Melbourne trains are slow, I'd assume you'd want a long stopping distance and time for a train
jakecopp 5y ago
Car stopping distances from 60mph/100kph are ~100 metres. 39,888 people died in the US in 2018 from car crashes. [1] Emergency braking distance of a TGV from 300 kmph (186 mph) is something like 3,500 m [2], carrying ~450 people. Moving block signalling systems take into account the current positions and velocities of both trains, and generate a maximum speed and braking curve. See [3] for the details, it's pretty fascinating systems design! [1]: https://en.wikipedia.org/wiki/List_of_countries_by_traffic-related_death_rate https://en.wikipedia.org/wiki/List_of_countries_by_traffic-r... [2]: https://railroads.dot.gov/sites/fra.dot.gov/files/fra_net/16334/1991_SAFETY%20RELEVANT%20OBSERVATIONS%20ON%20THE%20TGV%20HIGH%20SPEED.PDF https://railroads.dot.gov/sites/fra.dot.gov/files/fra_net/16... [3]: https://en.wikipedia.org/wiki/European_Train_Control_System https://en.wikipedia.org/wiki/European_Train_Control_System
iggldiggl 5y ago
Cars are driven based on relative braking distances, i.e. it is assumed that the vehicle in front of you will never come to a sudden stop. So you only need to keep a certain smaller margin to account for your reaction time (and some drivers ignore even that). This mostly works, but sometimes that assumption breaks down and you get something like a mass pile-up for example. Trains are held to higher safety standards, and so even modern train control system are usually based on absolute braking distance (and even when you want to space trains closer together regardless, as soon you need to throw a set of points between trains at a junction you're back to absolute braking distances, because as long as they aren't set safely towards one direction or the other, a set of points is effectively a stationary obstacle), i.e. there's always enough free space in front of the train to safely brake to a stop. This limits how closely trains can follow each other, and on urban railways it's normally the combination of dwell times (doors opening, passengers alighting and boarding, doors closing) and platform reoccupation times (wheels starting turning on the first train leaving the station to wheels stopping turning on the next train arriving at the station) that determine your minimum headway. (And for actually usable – as opposed to merely theoretical – capacity you also need to add at least a small amount of extra margin to compensate for possible dwell time variations and other day-to-day occurrences)
bluGill 5y ago
While it is true that cars regularly are less than a second apart, safety experts give 2 seconds as the minimum safe distance, and 3 is often recommended. You should be the different person who follows their advice, no matter how many people cut in front of you. Better late than dead, and highways are very dangerous.

alfiedotwtf 5y ago

Even though Melbourne trains are slow, I'd assume you'd want a long stopping distance and time for a train

jakecopp 5y ago

Car stopping distances from 60mph/100kph are ~100 metres. 39,888 people died in the US in 2018 from car crashes. [1] Emergency braking distance of a TGV from 300 kmph (186 mph) is something like 3,500 m [2], carrying ~450 people. Moving block signalling systems take into account the current positions and velocities of both trains, and generate a maximum speed and braking curve. See [3] for the details, it's pretty fascinating systems design! [1]: https://en.wikipedia.org/wiki/List_of_countries_by_traffic-related_death_rate https://en.wikipedia.org/wiki/List_of_countries_by_traffic-r... [2]: https://railroads.dot.gov/sites/fra.dot.gov/files/fra_net/16334/1991_SAFETY%20RELEVANT%20OBSERVATIONS%20ON%20THE%20TGV%20HIGH%20SPEED.PDF https://railroads.dot.gov/sites/fra.dot.gov/files/fra_net/16... [3]: https://en.wikipedia.org/wiki/European_Train_Control_System https://en.wikipedia.org/wiki/European_Train_Control_System

iggldiggl 5y ago

Cars are driven based on relative braking distances, i.e. it is assumed that the vehicle in front of you will never come to a sudden stop. So you only need to keep a certain smaller margin to account for your reaction time (and some drivers ignore even that). This mostly works, but sometimes that assumption breaks down and you get something like a mass pile-up for example. Trains are held to higher safety standards, and so even modern train control system are usually based on absolute braking distance (and even when you want to space trains closer together regardless, as soon you need to throw a set of points between trains at a junction you're back to absolute braking distances, because as long as they aren't set safely towards one direction or the other, a set of points is effectively a stationary obstacle), i.e. there's always enough free space in front of the train to safely brake to a stop. This limits how closely trains can follow each other, and on urban railways it's normally the combination of dwell times (doors opening, passengers alighting and boarding, doors closing) and platform reoccupation times (wheels starting turning on the first train leaving the station to wheels stopping turning on the next train arriving at the station) that determine your minimum headway. (And for actually usable – as opposed to merely theoretical – capacity you also need to add at least a small amount of extra margin to compensate for possible dwell time variations and other day-to-day occurrences)

bluGill 5y ago

While it is true that cars regularly are less than a second apart, safety experts give 2 seconds as the minimum safe distance, and 3 is often recommended. You should be the different person who follows their advice, no matter how many people cut in front of you. Better late than dead, and highways are very dangerous.