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Thread: Maximum traffic flow

  1. #1
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    Maximum traffic flow

    Does anyone know the speed at which roads generate the maximum traffic flow -- the most cars per hour? I'm assuming this varies for different kinds of roads, but are there any good benchmark numbers out there, or good articles? And is number necessarily near the speed limit, or can it below that?

  2. #2
    Cyburbian Jeff's avatar
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    This is not something that can be quantified on an internet message board without road geometry, % truck traffic, access points, intersections, etc.

  3. #3

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    Most transportation engineering books will have information on this... one is Principles of Highway Engineering and Traffic Analysis by Mannering/Kilareski. Have a look at the TRB's Highway Capacity Manual too.

    If you want a real rough/quick rule of thumb, regardless of the design speed, try 50-60 mph for 1800-2200 vehicles per hour per lane, at a level of service E.

    Hope this helps, until you have time to delve into the topic deeper.

  4. #4
    Cyburbian The One's avatar
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    Well.....

    No traffic whiz here....but I've been to many a seminar with many many smart people who say 35-45 miles per hour is the target speed for optimal traffic movement. In a heavy traffic high density urban environment, I'd tend to think the 35-45mph speeds will move more vehicles more efficiently, when considering the problems with controlled access highways in such densly populated areas. Interchange designs, toll roads, poorly timed lights....can you tell I lived in Broward County for three years

    Vehicle operation efficiency should still be closer to 50-55mph, so maybe that will be better for the environment in the long run??

    Rumpy......on your free time, you could solve this problem for us from your Desk Fort TM
    Skilled Adoxographer

  5. #5

    Check the HCM

    The Highway Capacity Manual http://www.highwaycapacity.com/images/hwycap_notes2.pdf deals with this extensively. Jeff is correct, there are several factors that determine this. However I don't think they calculate the Maximum traffic flow, you may have to plug in different speeds and back-calculate to find the value you are looking for. Several factors determine the optimal speed for a corridor.
    # of lanes
    divided/undivided lanes
    Grade/slope of road
    # of access points (roads/driveways)
    # of signals (or stop signs) and distance between them
    # of Semis

    The HCM also has software HCS 2000, which will give you level of service, A being the best F being worst. A is not practical unless you have free-flow and no access points. B or C is typical and desirable. This software is also very good for optimizing existing traffic signals and roadways. Check personnel in your engineering/traffic signal department/division.
    Who's gonna re-invent the wheel today?

  6. #6
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    I had come across the 35-45 mph figure "The One" had cited as well, which is why I had asked in the first place. But I wasn't sure if that was only for a certain kind of enviornment; I know that from loop data studies the maximum volume is achieved on LA freeways at 60 mph -- which is interesting in light of the number of people who drive above that to 'save time' but thus make the system slower for everyone...

  7. #7
    Cyburbian Jeff's avatar
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    This isn't a problem whose solution is a speed limit, or I'm not understanding the question.

    You are trying to solve for the maximum capacity of a segment of road? Or, just....at what speed does it work best?

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