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Svein submitted a new PF Insights post
An Analysis of Road Capacity
Continue reading the Original PF Insights Post.
An Analysis of Road Capacity
Continue reading the Original PF Insights Post.
This is cute from a physics point of view, but in real life, urban planners will tell you that streets with slower speed limits actually move more cars. A typical chart is on page 10 of this report. So in reality, speed limits do affect capacity, they just affect in the opposite way that you'd think.
Seems that article is basing those numbers on distance between cars, rather than time between cars. On southern California freeways, when congestion has not occurred, the morning commuters tend to achieve higher flow rates (number of cars per unit time) then the afternoon combination of commuters, shoppers, ... , with the higher flow rates correspond to higher average speed as noted in Svein insight. Other factors like morning commuters tending to follow each other more closely (for example flow rate around 1 car per 2.5 seconds rather than 1 car per 3 seconds), reacting and recovering quicker from temporary slow downs also helps. Once congestion occurs and speeds slow down, the flow rate decreases. This is obvious in the case where the speeds are so slow that the flow rate (per lane) becomes less than 1 car per 3 seconds.Greg Bernhardt said:Here is a comment from Reddit ... " urban planners will tell you that streets with slower speed limits actually move more cars."
I have found, purely anecdotally, that traffic flow on the freeways where I live (Seattle area) is better during the weekdays than on the weekends. My explanation for this is in line with your comment, @rcgldr, that daily commuters are more tuned into the traffic flow than the weekend drivers, some of whom drive the posted speed limit in the fast lanes.rcgldr said:Seems that article is basing those numbers on distance between cars, rather than time between cars. On southern California freeways, when congestion has not occurred, the morning commuters tend to achieve higher flow rates (number of cars per unit time) then the afternoon combination of commuters, shoppers, ... , with the higher flow rates correspond to higher average speed as noted in Svein insight. Other factors like morning commuters tending to follow each other more closely (for example flow rate around 1 car per 2.5 seconds rather than 1 car per 3 seconds), reacting and recovering quicker from temporary slow downs also helps. Once congestion occurs and speeds slow down, the flow rate decreases. This is obvious in the case where the speeds are so slow that the flow rate (per lane) becomes less than 1 car per 3 seconds.
Been done. It's called a train.anorlunda said:They say that the only way to do that is automated driverless vehicles that can operate with nearly zero distance between them.
Svein said:Been done. It's called a train.
That is not the main point. Drivers tend to over-react, and brake later and more than necessary.There are several reasons for queues forming. The most common is for cars trying to get onto a road which is already close to 100% full. Usually, this forces a car on the road to brake in order to keep the correct distance between cars. Since a speed reduction lowers the road capacity (Figure 3), this can suddenly cause the road to be more than 100% full.
Yes. I said that I described a steady-state situation. Your comment pertains to the dynamic response of cars on the road.mfb said:That is not the main point. Drivers tend to over-react, and brake later and more than necessary.
There are many simulations where you can study traffic flow for various different parameters, e.g. http://www.traffic-simulation.de/. Large decelerations increase the risk of a jam.
Yes. As I mentioned in the Insight, with a high speed limit a car goes faster from A to B when it has gotten onto the road. The problem lies in getting onto the road. If it is full (saturated), you cannot get onto it. If it is less than full, you must wait for an opening.rcgldr said:I'm wondering about the effect that speed has on how many cars there are on a freeway at any moment in time. At higher speeds, a commuter spends less time on the freeway (since the destination / freeway exit is reached sooner), which should result in fewer cars on the freeway at any moment in time.
This part of the dynamic response of the traffic. It is complex, that is why I invited in others to do that part..Scott said:The article also fails to note the "rubber-necking" affect. One one of two lanes is blocked, the capacity of the remaining lane drops as curious drivers what to know why they were delayed by 30 minutes.
That isn't my experience. People are always hurrying as a matter of principle in order to be on the platform before the next train leaves. They don't know the time of arrival so they go as fast as they can - pushing old gimmers like me out of the way in their hurry. Then, of course, statistically, they are as likely to have to wait the same time as I would (I could possibly have missed the train that they caught.)mfb said:In subway systems, most people don't care if they arrive a minute earlier or later - they have to wait for the train anyway. Some people care a lot because they might miss their connection if they are a minute late. So let those people rush, even if it means others (who have to wait anyway) are a bit slower.
Dumb, actually - and it shows the problem when taking situations to extremes. But I just don't care as I am remembering my hold up yesterday when driving through Southend.houlahound said:Smart
Restarting traffic gets very tricky in that case.sophiecentaur said:The inverse law of capacity with speed could be taken to ridiculous extremes if people got out of their cars and walked. The capacity would be in thousands but it would take you all day just to walk to and from work.
Ha Ha. They would have to get into different cars, at the other end. Human nature would be the only reason for not having a system like that.mfb said:Restarting traffic gets very tricky in that case.
Road capacity refers to the maximum number of vehicles that a road or highway can accommodate at a given time. It is typically measured in terms of vehicles per hour.
Road capacity is important because it affects the overall efficiency and safety of transportation systems. An inadequate road capacity can lead to traffic congestion, delays, and accidents.
Road capacity is determined by various factors such as the number of lanes, lane width, speed limit, type of road, and traffic control systems. It can also be influenced by external factors such as weather conditions and time of day.
There are several ways to increase road capacity, including widening the road, adding more lanes, implementing intelligent transportation systems, and improving traffic signal timing. These solutions can help reduce congestion and improve the flow of traffic.
Exceeding road capacity can lead to traffic congestion, longer travel times, increased air pollution, and a higher risk of accidents. It can also have a negative impact on the economy, as it can result in lost productivity and increased fuel consumption.