Are there any forensic civil engineers in the house?

[ACtrafficengr]

I'm a county engineer. One of our town highway superintendents asked us for technical assistance at a site of several fatal crashes. I have a hypothesis about what may have contributed to the latest crash, but I'd like to run it past a crash reconstruction or forensic specialist before making my recommendation. Would anyone be willing to lend an ear?

Thanks.

 

[dik]

If you can post a short description, I'm sure there is someone that will provide some information.

Dik

 

[chicopee]

If I were you, I would contact your state DOT. I know that they are out there. A TV program that I saw in the recent past involved a traffic accident between a police officer and secret service agents somewhere out west and ultimately a forensic CE absolved the police officer of the crash.

 

[JedClampett]

Chicopee, the show was Forensic Files. But like all of those shows, it involved a case 35 years ago (Reagan Administration)so I'm sure all the participants are long gone or retired.
But I think calling the Highway Patrol or DOT is the right path.

 

[dik]

Jed... ya, but they know where he lives...

Dik

 

[Ron]

AC...Your state DOT might not be a good place to go. They might be defensive if it turns out to be a geometry issue (which they typically control) or a maintenance issue (ditto).

Try one of the specialty firms such as Technology Associates (offices in New York and Florida) or Colony-Bell (Florida and other states)

Here are links...

Technology Assoc

Colony Bell

 

[ACtrafficengr]

I'm not looking to affix blame here, just help the town prevent it from happening again.

The crash site is a local road that some people like to drive on at excessive speed because they can get some air time on one of the crest vertical curves. In the most recent case, the estimated vehicle speed was 90 mph in a 30 mph zone.

I've designed a new v.c., but I strongly suspect the poor cross slope also contributed to the crashes. If I'm going to try to convince the highway superintendent to spend a sizeable hunk of his budget on truing and leveling HMA, I want to make sure I'm right.

The roadway goes from -4% excessive crown in the sag to dead flat at the crest in about 25 feet. It's -4% again in the next sag.

My hypothesis is the cross slope changes caused the vehicle to roll counterclockwise about its long axis (the right side going up) as it went over the crest, then clockwise as it came back down. The vehicle then went off the road to the right, and, perhaps because the driver overcorrected, went off the left side of the road into a tree.

So, does that sound like a plausible chain of events?

 

[dik]

... and that transition is in 1/5 second at 90, approx, if I understand the description... maybe for F1 drivers...

Dik

 

[Ron]

Have you considered "calming" strips?

 

[ACtrafficengr]

I'm not familiar with the term. Do you mean rumble strips? Sleeping policemen?

 

[Ron]

AC....they are longer (wider) than rumble strips and not as annoying to non-speeders. Usually 4-6 feet long and raise pavement 4-6 inches.

 

[chicopee]

If nothing above helps, speed traps and heavy fines imposed by your county judge would help; also speeding records would affect violators' insurance rates and perhaps jeopardize driving privileges.

 

[Spartan5]

Point of order here, but I don't think the highway superintendent should be expending any part of his budget to be correcting an accident where the primary cause was traveling 3X the posted speed limit.

Is your primary objective to modify the VC such that you can't go airborne if you hit it at near triple digit speeds?

 

[ACtrafficengr]

Spartan5, I don't disagree, but it wasn't an isolated occurrence. It is apparently a rite of passage or something among the local high school students. Another kid posted a video of a high speed run down the road last week.

So, the super is in a spot where the town supervisor, town board, and local papers wants him to do something about it. I think highway superintendent may still be an elected position in his town, so keeping his job may also play a role.

The goal is to discourage the risky behavior, so, yes, we basically want to be Oscar the Grouch and take away their fun.

 

[ACtrafficengr]

So, anyway, for the existing crest, K < 8. Lengthening the VC to provide K=19 as per AASHTO for 30 mph would require 105 feet of full depth reconstruction. a 35 mph crest would be 160 ft long. Would there be much benefit to going beyond that?

 

[LittleInch]

Most cars are simply not designed to"get some air"and are not balanced either axially or side to side and will dip and hit one side before the other and start to rotate ever so slightly. The camber changes won't help but different cars will react differently. A set of bends is the only thing to prevent people getting the straight line speed required.



Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[ACtrafficengr]

LittleInch, that would require buying right-of-way, and arguably make the road more fun. "Woohoo! It was a drag strip. Now it's Watkin's Glen!"

 

[LittleInch]

I didn't say it was easy but if you watch even professional rally drivers and cars getting it wrong on air jumps leveling out the road horizontally isn't really going to help much. IMHO.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Spartan5]

Couldn't you just add a couple of speed bumps? You know, the sort that will scrape all their "ground effects" trim loose and pop the spoilers off the back of their car if they hit them at more than 25 or so?

 

[TerryScan]

When traffic calming design is "more fun" we are approaching limits of what one can do with engineering vs. enforcement.

Personally, I DO believe that making the cross slope work better/safer for a landing after "catching air" will only encourage the behavior.

I would look into other calming measures as well.

http://www.ite.org/traffic/tcdevices.asp

Perhaps a combination of measures (alternate choke and center median, with humps before/after high point?) would be needed to create a detterant.

I deal with many municipalities which keep wanting wider streets these days and restricted street parking in subdivisions. Vehicles go considerably slower when streets are narrow with parking. Sometimes it doesn't have to be as complicated.