Using a cell phone video to detect traffic speeds | Hacker News Reader

Using a cell phone video to detect traffic speeds

github.com

I always found the American system of speed limits ending in 5 a bit weird. If you have a modern car that has a speedo going to 150 or maybe even 180 miles an hour, the distance between 20mph and 30mph on the dial is 1-2mm, so tiny that it's really hard to stay in that spot right between them to follow a 25mph limit

> ...the distance between 20mph and 30mph on the dial is 1-2mm, so tiny that it's really hard to stay in that spot right between them to follow a 25mph limit

You are doing it wrong. You do not need to stay "in that spot" - you only need to ensure you are not speeding. For most drivers, this is not hard, and if you are not able to do it, perhaps you should not be driving.

I especially like how you skipped past the entire point of my post - that speed limits ending in 5 make little sense, and that's the point I wanted to discuss. Whether I can stay below the limit or not was neither mentioned, nor has any impact on this discussion.

This is a fascinating exercise with a disappointing argument and pre-determined conclusion.

The writer notes that higher speed increase likelihood of an injury in the case of an accident. There is no comment on the factors that influence the likelihood of an accident and how these change with speed. By this logic, every road should have a 20mph limit - clearly this would do more harm than good overall.

A better analysis would take into account the utility of all relevant parties (including drivers). This would mean superior solutions such as those that maintain car speed as well as improve cyclist/ped safety could be considered rather than simply constructing an argument to fit one solution the writer has proposed without any evidence that it would even work.

Engineering from first principle tells us that roads where cars interact with unprotected humans should have a limit of 20 mph. I'm not sure if this is such a road but overall, because of physics, speed isn't really up for debate.

Edit: Also really sad to see this get removed, flagged(?), from the front page. It was a really cool hack. HN is all about BS and not technology these days it seems.

Unfortunately many roads in London have 20mph speed limits now.

They do have a purpose at certain times of day but the inefficiency of doing 20mph when the road, cycle lanes and pavements are largely empty is disappointing.

Reply on news.ycombinator.com