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Thread: Design speed versus actual speed

  1. #1
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    Design speed versus actual speed

    This is related to the Texas speed limit debate a while back, but I'm curious about this issue of design speeds on roads that result in the driver constantly breaking the law as they drive what feels 'natural.'

    How did this get to be Scripture in traffic engineering, that design speed should be so much higher? Is there a proven safety benefit, or is it a comfort thing? I know narrower roads are being tried in some places as a reaction to excessive speeds -- the 'self organizing' road its called in Europe.

    Has anyone seen any good articles/books on this issue, pro or anti-higher design speed?

  2. #2
    Cyburbian jordanb's avatar
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    It comes from applying engineering principles to traffic.

    If you're building a highway and you're trying to figure out how strong it'll be, you find out how heavy a truck you want to have on it and then you design the road to handle a truck that weighs 120% what that truck weighs. The extra 20% is the safety factor, it comes into play if there's something the engineer overlooked that makes the road weaker than he planned or if someone sneaks a heavier truck onto it.

    So Traffic Engineers figured, if you want to make a road safe for driving 55 mph on, well, better make it safe to drive 66 mph on (or whatever the safety factor is) so that when someone speeds, they don't instantly become unsafe. The problem is that everyone drives at the speed where they, personally, feel "safe," regardless of what the community has determined to be the proper speed for that road. So when you put in a safety factor when designing a road you just end up making it so everyone drives faster than was intended.
    Reality does not conform to your ideology.
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  3. #3
    Jordan is correct. It is the safety factor thing. We are taught that design speeds add in this "safety" factor for those vehicles that don't follow the speed limit. I'm not saying designers anticipate that everyone breaks the speed limit, but it is, in part for emergency vehicles which regularaly exceed the speed limit too.
    The feds (FHWA) sets the rules and if you want (need) their grant money, you will follow them: 12' wide drive lanes, 10' multi-use paths, etc., etc.
    Who's gonna re-invent the wheel today?

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    Cyburbian Jeff's avatar
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    Design speed is used to shape the geometry of the road. In standard engineering practices, it is recommended to use as high a design speed as the topo, site, etc allows.

    engineers design for safety, higher design speeds allow for safer design. It is not the engineers responsibilty to enforce traffic laws or shape peoples behaviour.

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    Cyburbian njm's avatar
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    If I remember correctly, normal designs for high-speed roads is 75 for 55mph limit and 90 for 65mph limit. However, I took transport engineering some 5 years ago, so I might be off...
    What luck! A random assemblage of words never sounded less intelligent.

  6. #6
    Cyburbian Budgie's avatar
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    Quote Originally posted by Jeff
    engineers design for safety, higher design speeds allow for safer design. It is not the engineers responsibilty to enforce traffic laws or shape peoples behaviour.
    This may be true for high speed traffic where pedestrians are not in the mix. "Higher Design Speeds Allow for Safer Design" is not true in residential areas. People will drive at the speed they are comfortable at and comfort is based on a preceived (real or unreal) level of safety. If you build a race track, people will race. Yes, law enforcement is necessary, but design is very very important.

    Have you every riden a bicycle down a hotel hallway? You feel as if you are traveling much faster than if you were outside. The feeling enclosure makes you slow down, which means you are moving at a safer speed.
    "And all this terrible change had come about because he had ceased to believe himself and had taken to believing others. " - Leo Tolstoy

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    Cyburbian Jeff's avatar
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    You are confusing terms.

    Design speed is merely a factor in the design of the highway, it is not a tool to curb peoples behavior.

  8. #8
    Quote Originally posted by Jeff
    You are confusing terms.

    Design speed is merely a factor in the design of the highway, it is not a tool to curb peoples behavior.
    Jeff, I think that is Budgies point (correct me if I'm wrong here Budgie), is that in order to curb people's behavior, he thinks that the design speed is actually "over-design" and that the design speed should be minimized - or the same as the posted speed in order to slow or calm traffic in residential areas at least.
    Budgie, there are curves in residential areas too that require embankments and the design speed needs to apply to them also. The design speed is so that a car won't go flying off a roadway (mostly in a curve) because they go 1 mph over the posted speed limit. In Ohio on residential streets, the design speed is 15mph over the posted speed limit. Keep in mind that design speed primarily affects curve design, versus straight roadways, which are affected minimally (more pavement markings than anything) by design speeds.
    TEST: Try taking an off ramp at 10 mph over the posted speed limit, then try taking it at 30 mph over the posted speed limit. The 30 mph is exceeding the design speed and you likely won't be able to negotiate the curve and rub some guardrail, whereas the 10mph over this curve will still be fairly easily managable.
    Who's gonna re-invent the wheel today?

  9. #9
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    engineering

    "It is not the engineers responsibilty to enforce traffic laws or shape peoples behaviour."

    It may not be their responsibility, but isn't that what they are de facto doing in designing highways that 'seem' slower than they are?

    Sure, engineers overdesign for safety, but what good is that if the extra margin is simply filled up or even replaced, with too much weight, too much speed, etc.

  10. #10
    I took a travel demand modeling course where the experts teaching explained using a predicted free-flow speed of 10% higher than the posted speed limit. So, not only are highways often used near their actual design speed, but a realistic study of their capacity includes the higher speeds explicitly.

    You know those square engineers drive the speed limit themselves, though.

  11. #11
    Cyburbian DetroitPlanner's avatar
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    Freeway engineering is a fascinating thing. I was told by one of the old engineers that worked on many of the interstates in my young planner days that the roads were built so that if both the car and the road were in top condition, that the car would literally drive itself through the slight curves in many places at some speed like 75 mph. I dunno if I'd want to take my hands off the wheel while taking a curve at 75 mph!
    We hope for better things; it will arise from the ashes - Fr Gabriel Richard 1805

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