Traffic Simulation • Hacker News Board

Re: Traffic Simulation

#21

Post by takk309 » Sun, Jun 12, 2016, 3:37 AM UTC

sandworm101 wrote: post #19

takk309 wrote: post #18

Earlier quoted context omitted.

Traffic engineer here. Generally speed limits are set by measuring vehicle speeds for about a week. The 85th percentile speed is set as the speed limit. The reasoning behind this is the assumption that most people will travel at a speed that is reasonable for the facility. Of course, politics often becomes a factor and the speed is changed to fit those pressures. Other aspects that can be taken into account when sett…

That's the textbook answer. The reality is that most jurisdictions have some fixed limits depending on the class of road. Parks and such, 30kph. Normal streets 50. Highways 80, with 90/100 where there are no ramps/intersections/driveways/people. If we believed in the 85th percentile then we would have lots of different apparently random limits. Instead we have limits based on polices very much separate from road cond…

You are correct, my response is the text book answer. Often the 85th percentile is rounded to the nearest speed that matches the fixed limits. Special speed zones, schools, definitely need to exist for obvious safety reasons.

Re: Traffic Simulation

#22

Post by reitanqild » Sun, Jun 12, 2016, 8:57 AM UTC

takk309 wrote: post #21

sandworm101 wrote: post #19

Earlier quoted context omitted.

That's the textbook answer. The reality is that most jurisdictions have some fixed limits depending on the class of road. Parks and such, 30kph. Normal streets 50. Highways 80, with 90/100 where there are no ramps/intersections/driveways/people. If we believed in the 85th percentile then we would have lots of different apparently random limits. Instead we have limits based on polices very much separate from road cond…

You are correct, my response is the text book answer. Often the 85th percentile is rounded to the nearest speed that matches the fixed limits. Special speed zones, schools, definitely need to exist for obvious safety reasons.

Thanks for chiming in. If you still look at this thread: I guess you live in the US? (Around here it does not seem to work this way.)

Re: Traffic Simulation

#23

Post by takk309 » Mon, Jun 13, 2016, 3:32 PM UTC

reitanqild wrote: post #22

takk309 wrote: post #21

Earlier quoted context omitted.

You are correct, my response is the text book answer. Often the 85th percentile is rounded to the nearest speed that matches the fixed limits. Special speed zones, schools, definitely need to exist for obvious safety reasons.

Thanks for chiming in. If you still look at this thread: I guess you live in the US? (Around here it does not seem to work this way.)

Yes, I do live in the US. Even here there is a lot of variation on how speed limits are set.

A good example happened near my home recently. A road that connects a small town (population 8,000) to a larger town (population 40,000) was the subject of quite a lot of debate. The road is a rural two lane roadway that sees about 10,000 vehicles per day. Speed studies were performed by three different groups (the small city, the large city, and the State Department of Transportation). Each of the groups determined their own speed limit recommendation, each was different. Ultimately, the State had final say on the matter and split the difference between the three recommendations. The State had originally wanted the highest speed limit of the three. The two towns wanted lower speed limits citing safety concerns and a desire to force alternate routes that would take traffic through areas with better infrastructure for the traffic load.