3 ms·

I live somewhere with lots of roadside speed signs (they flash speed in red if you're speeding, green if under). In the tens of cars I've driven/ridden here, th

I live somewhere with lots of roadside speed signs (they flash speed in red if you're speeding, green if under). In the tens of cars I've driven/ridden here, the speedometer is always within a small range (±2km) of what the road sign says

kevin_thibedeau 7y ago
radar detectors always read lower than your true speed since they can't be positioned directly in your line of travel.
- SECProto 7y ago
  In the situation I experience on my commute daily: I'm 40m back and the detector is 3m perpendicular off my line of travel (reasonable approximation, as they're on a roadside light standard, the lane width is 3.5m, and the car is centered in the lane), that is a very minor discrepency. On the order of 0.2 km/h
perl4ever 7y ago
There are some weird speed signs near me. One that particularly irritates me is set up to flash if you're over 25 mph. But the speed limit is 30! There's also a flashing sign by a school, that is left on at times that the regular sign specifically says the school speed limit is not in effect.
vanderZwan 7y ago
The thing is that you have to look at this from the perspective of the expected largest difference between different cars, not one speedometer and actual speed. If two cars both say they drive at 50 km/h, and one is actually 45 km/h and the other 55 km/h, that is quite a bit relative margin of error. And we only need two cars like that in the whole group to start causing problems.
- SECProto 7y ago
  But I don't think you have to look that hard at some technical error causing this variance. Drivers speeds vary between 40 and 60 pretty regularly on residential roads. Having a speedometer error doesn't even have to enter into the conversation. If you can't design a green wave to be tolerant of varying speeds, then it isn't useful.

3 ms·

I live somewhere with lots of roadside speed signs (they flash speed in red if you're speeding, green if under). In the tens of cars I've driven/ridden here, th

by SECProto 7y ago

kevin_thibedeau 7y ago
radar detectors always read lower than your true speed since they can't be positioned directly in your line of travel.
- SECProto 7y ago
  In the situation I experience on my commute daily: I'm 40m back and the detector is 3m perpendicular off my line of travel (reasonable approximation, as they're on a roadside light standard, the lane width is 3.5m, and the car is centered in the lane), that is a very minor discrepency. On the order of 0.2 km/h
perl4ever 7y ago
There are some weird speed signs near me. One that particularly irritates me is set up to flash if you're over 25 mph. But the speed limit is 30! There's also a flashing sign by a school, that is left on at times that the regular sign specifically says the school speed limit is not in effect.
vanderZwan 7y ago
The thing is that you have to look at this from the perspective of the expected largest difference between different cars, not one speedometer and actual speed. If two cars both say they drive at 50 km/h, and one is actually 45 km/h and the other 55 km/h, that is quite a bit relative margin of error. And we only need two cars like that in the whole group to start causing problems.
- SECProto 7y ago
  But I don't think you have to look that hard at some technical error causing this variance. Drivers speeds vary between 40 and 60 pretty regularly on residential roads. Having a speedometer error doesn't even have to enter into the conversation. If you can't design a green wave to be tolerant of varying speeds, then it isn't useful.