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Comment by kjkjadksj | Hacker News Reader

I've seen something like this take place right in front of me. The light I guess was on a magnetic signal and not a regular cycle. The car ahead of me inched too far ahead of the magnet strip, I was too far from it. I had nothing to do that day and was curious how this would play out so I just sat there and gave them sufficient room to back up if they realized it. We must have waited a good 5 minutes with cars piling up behind us before that first car backed up, triggered the light in 20 seconds, then off we all went. There's plenty of social engineering potential out there to cause a lot of confusion, consternation, even chaos, but seems like the sort of hacker that would get their kicks off that is an extremely rare breed, or these sorts of things would be happening all the time for all sorts of parts of our life.

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I used to bicycle along a route where there was a left turn like this, the light wouldn't detect my bicycle. If I sat in the left turn lane on my bike (like I'm supposed to if I'm turning left) and the car behind me stopped a reasonable distance from me the light would never change. This is a street that terminates into the cross street, so there's no one across from us to trigger it, no one going straight to trigger it, and right turners would clear out too fast to trigger it. If I was malicious I could easily have grown a pretty absurd line of cars and buses behind me.

I no longer bike through that intersection (because I don't need to very often, and because I found some nice side streets that happen to take me around it), but I doubt it's changed in the years since.

I always stopped out in front of the line, and gestured for the car to move up to trip the sensor for lights like this.

Which 90% of the time leave the drivers puzzled and wondering if you are about to do them any harm if they get too close. Been there, done that.

It's also usually possible to trigger the green light by using the button that is at the crosswalk (inteded to be used by pedestrians when they need to cross).

You can attach a small permanent magnet somewhere on your bike's frame and it will trigger the ground loops. They're a common add-on for motorcycles, too.

that doesn’t work for the majority of these sensors as they rely on inductance

In that case, simply install a chunk of steel under the bicycle - 150kg or so should be sufficient.

I found steel rims to almost always work as long as you put it right over the middle line. In CA they often paint a little bike where you need to put it.

A simulated inductance circuit would do

Is there a way to visually tell what kind of sensor loop you're dealing with?

I can verify that a magnet makes the difference for my steel-framed e-bike, but really there's only half a dozen lights in town (Gilroy, CA) that I frequently use.

As far as I know all of the sensors that are embedded in the pavement use inductance.

I remember with one such intersection that one strategy was to lay the bicycle sideways on the pavement to trigger the sensor.

Most lights around here have three visible coils to prevent this - and even if nothing triggers it they change once every few minutes anyway.

> and even if nothing triggers it they change once every few minutes anyway

This isn't 100% true. There are many small, rarely traveled, residential streets (i.e. only the people who live on the small street come/go from there) that feed into high traffic, multi-lane arteries.

Parent comment's scenario is pretty common due to detection based switching. 99.99% of the time this is desirable because it keeps the artery flowing.

If there's a crosswalk button across the artery, you'll many times find a person running out of their car to hit that to force the light change.

Are these traffic signals all programmed with controls unique to each intersection?!

Certainly many places have signals that are coordinated for several blocks, but in my experience, more often than not, intersections appear be controlled by local sensors.

Agreed, generally only in metro areas with the few miles leading in/out do you find coordinated traffic systems. Vast majority of road length is in suburbs where local sensors dominate.

Had a few friends who went to civil eng and two eventually joined city planning. Was fun to discuss their problems. Lack of funding really was the common thread. It's always lack of money.

The solution to this issue is that that crossing should not exist or should be a right-turn only crossing.

This is also the perfect case for roundabout (one major artery and one or more low volume streets)

Pretty expensive solution to a simple detection problem? I feel like this is optimizing for the wrong thing.

Not really, the expensive thing is wide roads and low density sprawl.

ADS-B (so-called “secondary radar”) (actually aircraft telemetry) is broadcast with no encryption or authentication of any kind. Anyone can get on 1090MHz and transmit any packets they like (until the FCC and FAA and FBI arrive). You could make a hundred ghost planes appear traveling perpendicular to the runways of a major airport at 200 feet AGL.

For some reason, nobody does this.

This is one of these systems where you need somebody at the location you want to hack, and that makes things a lot harder from an opsec / legal perspective.

It is pretty trivial to detect where a malicious radio signal is coming from. Combine that with the requirement that it must come from the country being hacked, where police is presumably easy to deploy, and you get an almost-unhackable system in practice, despite many theoretical security flaws.

Compare that to the internet, where detecting the true origin of traffic (assuming the attackers are using VPNs and/or TOR) is very difficult, and even if you do so, you still have to convince some foreign law enforcement agency to prosecute. This is non-trivial with unfriendly countries like Russia, and actually impossible if the attacker is the nation-state itself.

This is only true for very broad definitions of "the location you want to hack"

Does anyone else not take advantage of magnetic loops to get an advanced left turn?

The traffic signals in my area are often setup to give advance green if there are two or more cars stopped. Every so often, I’ll stop short a car length from the stop line.

Not recently but I recall an intersection where I used to let my car roll back down the hill a bit in neutral if no one was behind me to quickly trigger the light. I didn't understand the mechanism at the time, just that I could reliably get the trigger doing this.

> potential out there to cause a lot of confusion, consternation, even chaos, but seems like the sort of hacker that would get their kicks off that is an extremely rare breed

Definitely not a rare breed. They're just busy elsewhere

Then there are the people who don’t seem to realize that the flashing red stoplight is never going to start flashing green.

The sensors use an inductive loop, not a magnet. It does interact with sufficiently large metal objects, so I can see why folks call it magnetic.

These generally have to be tuned for bicycles and even motorcycles, to detect the much smaller mass.

They're supposed to be tuned but often aren't, as many motorcyclists can attest to

Further the muni often lacks resources to fix this in a timely manner after reporting. In some cases you see states effectively throw up their hands and say "if it doesn't detect you can just run the red"

https://www.advrider.com/f/threads/dead-red-traffic-signal-l...

Good luck with that on a bicycle with carbon frame, rims and other components. Some bikes haven't that much metal anymore, with even cranks, pedals, levers and derailleurs being made partly of composite materials.

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