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Old 11-22-10, 03:11 PM   #1
invisiblehand
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Confessions of a recovering engineer

http://www.strongtowns.org/journal/2...-engineer.html

Excerpt ...

When the public and politicians tell engineers that their top priorities are safety and then cost, the engineer's brain hears something completely different. The engineer hears, "Once you set a design speed and handle the projected volume of traffic, safety is the top priority. Do what it takes to make the road safe, but do it as cheaply as you can." This is why engineers return projects with asinine "safety" features, like pedestrian bridges and tunnels that nobody will ever use, and costs that are astronomical.

An engineer designing a street or road prioritizes the world in this way, no matter how they are instructed:

1.Traffic speed
2.Traffic volume
3.Safety
4.Cost

The rest of the world generally would prioritize things differently, as follows:

1.Safety
2.Cost
3.Traffic volume
4.Traffic speed

In other words, the engineer first assumes that all traffic must travel at speed. Given that speed, all roads and streets are then designed to handle a projected volume. Once those parameters are set, only then does an engineer look at mitigating for safety and, finally, how to reduce the overall cost (which at that point is nearly always ridiculously expensive).
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Old 11-22-10, 03:53 PM   #2
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Is there an inverse relationship between safety and traffic speed?

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The rest of the world generally would prioritize things differently, as follows:

1.Safety
2.Cost
3.Traffic volume
4.Traffic speed
I'm sure that if asked, people would prioritize safety in the abstract, but if you get them on the road I have a feeling that getting where they're going quickly would trump all other concerns. I can't think of a better way to seriously piss off drivers than this idea. And apparently it's not clear that lowering speed limits actually increases traffic safety.
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Old 11-22-10, 04:14 PM   #3
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Originally Posted by mountainjesus View Post
And apparently it's not clear that lowering speed limits actually increases traffic safety.
I have never heard of a traffic fatality who was struck at 1mph. I very much doubt that 1mph crashes are more common and more fatal than 70mph crashes. Speed plays a role.
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Old 11-22-10, 04:23 PM   #4
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Thanks for the post, it's interesting.
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Old 11-22-10, 04:26 PM   #5
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Is there an inverse relationship between safety and traffic speed?



I'm sure that if asked, people would prioritize safety in the abstract, but if you get them on the road I have a feeling that getting where they're going quickly would trump all other concerns. I can't think of a better way to seriously piss off drivers than this idea. And apparently it's not clear that lowering speed limits actually increases traffic safety.
First of all, you're a troll. This thread isn't about that at all. The article doesn't even discuss speed beyond where it ranks in the priority list. He doesn't say one necessarily comes at the cost of the other at all. And he mentions speed limits once, and with it he includes a caveat about their safety.


Second, I think you have a point buried in there. I suspect your typical person wants:
1. Safety for non-motorists where they live and play.
2. Speed for motorists where they don't.

Given people who live at any location, chosen at random, maximize both variables for all. Good luck .
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Old 11-22-10, 04:31 PM   #6
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Originally Posted by invisiblehand View Post
http://www.strongtowns.org/journal/2...-engineer.html

Excerpt ...

When the public and politicians tell engineers that their top priorities are safety and then cost, the engineer's brain hears something completely different. The engineer hears, "Once you set a design speed and handle the projected volume of traffic, safety is the top priority. Do what it takes to make the road safe, but do it as cheaply as you can." This is why engineers return projects with asinine "safety" features, like pedestrian bridges and tunnels that nobody will ever use, and costs that are astronomical.

An engineer designing a street or road prioritizes the world in this way, no matter how they are instructed:

1.Traffic speed
2.Traffic volume
3.Safety
4.Cost

The rest of the world generally would prioritize things differently, as follows:

1.Safety
2.Cost
3.Traffic volume
4.Traffic speed

In other words, the engineer first assumes that all traffic must travel at speed. Given that speed, all roads and streets are then designed to handle a projected volume. Once those parameters are set, only then does an engineer look at mitigating for safety and, finally, how to reduce the overall cost (which at that point is nearly always ridiculously expensive).
The proposed order of priority doesn't work. Engineering structures, of whatever nature, are intended to do something. To put safety first simply means that it would be best to have no road at all, for that would eliminate all road accidents.
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Old 11-22-10, 08:38 PM   #7
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There is a growing realization in the traffic engineering community that flat, wide, straight roads are not always safer in the general sense. So the situation is changing fairly rapidly.

It really seems to me that the speed limit should be self-enforcing. I.e., the speed limit should be about what people feel is appropriate for the road. I ride on residential roads that are 3 1/2 lanes wide (if nobody is parked on them, which is common) and find myself unsurprised that the traffic is going 50 in a 25 zone.
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Old 11-22-10, 08:50 PM   #8
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There is a growing realization in the traffic engineering community that flat, wide, straight roads are not always safer in the general sense. So the situation is changing fairly rapidly.

It really seems to me that the speed limit should be self-enforcing. I.e., the speed limit should be about what people feel is appropriate for the road. I ride on residential roads that are 3 1/2 lanes wide (if nobody is parked on them, which is common) and find myself unsurprised that the traffic is going 50 in a 25 zone.
Self-enforcing speed limits is what we have now: the actual speed limit is what the 85th percentile driver is going. My state DOT explicitly states that drivers will travel at safe speeds. Such sentiments are so contrary to the facts that I could cry. Somehow motorists that kill 40,000 and injure 4 million people per year are safe? That's just nuts. I suspect the only thing that would cause motorists to travel at safe speeds is zero-tolerance enforcement and huge consequences for infractions (loss of car, loss of license, fines of 5-10% annual income).
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Old 11-22-10, 09:20 PM   #9
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by self-enforcing speed limits, I mean designing roads in such a way that a motorist recognizes that they should go the speed limit. There are many ways of doing this, such as traffic tables, roundabouts, and others. Putting a road with 14 foot lanes and clear sightlines through a residential area is tantamount to telling motorists that speeding is ok, even if the speed limit is 20mph. Draconian enforcement, stop signs, and illogical speed limits have been tried and I can show you 10 roads here in State College where these methods have failed.
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Old 11-22-10, 09:25 PM   #10
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The only success stories of speed tables and bulb-outs in reducing speeds where I live have worked by making the route such a pain that the speeders are choosing an alternate route. It is a rob Peter to pay Paul strategy, since the city won't put them everywhere. I'm not a big fan of such measures since they seem to generally take away the line one would ride a bike in and force multiple poorly designed merge situations.
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Old 11-22-10, 10:26 PM   #11
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I avoid road with humps, tables and bulb outs. Too much hassle and the motorists on them are in competition for the remaining too little space left over.
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Old 11-22-10, 10:27 PM   #12
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Draconian enforcement, stop signs, and illogical speed limits have been tried and I can show you 10 roads here in State College where these methods have failed.
I would guess because the enforcement wasn't there. There are two towns near where I grew up that had reputations of giving tickets if you went 1 mph over the limit. People would speed to the town, then they would slow down to the limit through those towns.
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Old 11-22-10, 10:38 PM   #13
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The proposed order of priority doesn't work. Engineering structures, of whatever nature, are intended to do something. To put safety first simply means that it would be best to have no road at all, for that would eliminate all road accidents.
I didn't take him literally since he is writing for the public. I thought of it as more as describing a weighting function (none of the parameters are zero nor infinity) in the context of a constrained optimization problem.
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Old 11-22-10, 10:44 PM   #14
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The only success stories of speed tables and bulb-outs in reducing speeds where I live have worked by making the route such a pain that the speeders are choosing an alternate route. It is a rob Peter to pay Paul strategy, since the city won't put them everywhere.
in general, I think this is what you want. Most towns have a system of arterials and parallel neighborhood streets. If there is no penalty to go on the quieter neighborhood streets, people will speed down them. If traffic is calmed properly, then these quieter streets should be fine for cycling, leaving the arterials for motorized traffic. As it is now, typically there is a stop sign at every block in the quieter streets, which are commonly ignored by everyone and make the streets unsuitable for bicycle traffic.
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Old 11-23-10, 09:03 AM   #15
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Nobody has ever tried this: End prosecution of car thefts that happen on or near roads with chronic speeding problems. Tell drivers that when they behave, we'll treat auto theft like a crime again. Refuse to protect potential homicide weapons, in other words.
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Old 11-23-10, 11:35 AM   #16
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In a discussion about upcoming AASHTO guidelines one note was that 10' lanes are safer then 12' and have the same capacity. But stressing speed demands 12'. Side trees and street parking were also noted to slow speeds and improve safety (rarely done.) So in short we design roads so they are very tolerant of (encourage) speed over and above the posted speed limit.
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Old 11-23-10, 12:16 PM   #17
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Draconian enforcement, stop signs, and illogical speed limits have been tried and I can show you 10 roads here in State College where these methods have failed.
That's because it wasn't draconian. Draconian enforcement would involve very high penalties, which we don't have, for minor infractions. So speed 1mph over and we, oh let's say, take your car and sell it for state funding.
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Old 11-23-10, 01:56 PM   #18
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OP is spot on, except this isn't the way engineers work everywhere, just in the US and the rest of the english speaking world (UK, OZ, NZ and Canada)
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Old 11-23-10, 02:06 PM   #19
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The proposed order of priority doesn't work. Engineering structures, of whatever nature, are intended to do something. To put safety first simply means that it would be best to have no road at all, for that would eliminate all road accidents.
I think the priorities are supposed to be applied to a basic road structure (2-lane, 3 lane, 4 lane, shoulder/no shoulder, etc).

But you're right that some amount of traffic volume and speed has to be designed for above safety. I think the point was that even after some reasonable amount of volume is designed for, and some reasonable speed, they continue to work for more volume and speed. Sometimes they do so to lower cost, because they don't want to redesign a wider, safer and faster road but they need more volumne. Sometimes they do so simply because of congestion.
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Old 11-24-10, 06:38 PM   #20
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Originally Posted by unterhausen View Post
by self-enforcing speed limits, I mean designing roads in such a way that a motorist recognizes that they should go the speed limit. There are many ways of doing this, such as traffic tables, roundabouts, and others. Putting a road with 14 foot lanes and clear sightlines through a residential area is tantamount to telling motorists that speeding is ok, even if the speed limit is 20mph. Draconian enforcement, stop signs, and illogical speed limits have been tried and I can show you 10 roads here in State College where these methods have failed.
I can't remember where I first saw it, but the idea that adding extra stop signs to slow traffic down results in increases peak speeds and increased travel times. I lived in Dallas in the mid 80's, and 4 lane roads through residential areas were the norm. 50 MPH was not uncommon during rush hour on those roads, but everyone slowed down to 20 in the school zones because of the stiff fines. It took a while to get used to driving down there!
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Old 11-25-10, 05:02 AM   #21
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Thanks for the link.

The only thing that struck me as strange is the thought of using auto parking as traffic calming- sounds to me like keeping kids off your lawn by hiding sharp objects in the grass.
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Old 11-26-10, 05:53 PM   #22
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... competition for the remaining too little space left over.
This is exactly the "market" pressure that should be successful in driving people to biking or public transit. It makes me sick to see my tax money spent by the basket-full on expanding roads to make it easier to cage at higher speeds.
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Old 11-27-10, 04:41 PM   #23
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This is exactly the "market" pressure that should be successful in driving people to biking or public transit. It makes me sick to see my tax money spent by the basket-full on expanding roads to make it easier to cage at higher speeds.
Absolutely -- we must stop subsidizing private motor vehicle travel. If gasoline cost around $8 per gallon, to cover the full societal cost of driving, we would see more people opting for human-powered transportation and public transit.
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Old 11-27-10, 09:49 PM   #24
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Absolutely -- we must stop subsidizing private motor vehicle travel. If gasoline cost around $8 per gallon, to cover the full societal cost of driving, we would see more people opting for human-powered transportation and public transit.
Calculating the "full societal cost of driving" is fraught with complexities. And most arguments I have seen that attempt to use this basis also ignore the "full societal benefits of driving." In short, most of such arguments are basically anti-motoring arguments. I think that little credence should be placed on such arguments.
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Old 11-27-10, 10:01 PM   #25
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What exactly are 'the full societal benefits of (non-commercial/emergency) auto use'? The ability to carry a home theater system around with you at all times? Or are obesity, debt, and a daily reliance on fossil fuels 'benefits'?
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