A question about freeway barriers

Along the divided highways throughout Texas, TxDOT has taken to installing barriers on the grassy median. Sometimes, these are concrete walls. Mostly, though, they are barriers constructed of three steel cables running through a series of posts. Here is an example of the barrier on a stretch of I-10.

What I don’t get is the location of the barrier on the median. It is always about one third of the way from one side or the other. It is never down the center of the strip, equidistant from both sides of the traffic lanes. In the above example, you can see that the barrier is closer to the westbound lanes than to the eastbound lanes. At seemingly random intervals, the barrier is switched to where it is closer to the eastbound lanes.

Why don’t they put the barrier in the center of the freeway?

Maybe because the centre of the median is the lowest point, and they want to both keep that area clear and make sure that the barrier is as high as possible?

I’m going to guess it’s something location specific. I know it I’ve seen the cables* down the center in some areas. The only time they drift off center is when they are making a spot for police to make U-Turns. One possibility I can think of is if the center of the median is lower. If it’s too low I could see a car going at a high speed making it over the top of them.

*Interesting factoid: The cables aren’t really anchored very tightly to the posts except at the ends when they are buried in concrete in the ground. They’re designed to catch cars rather then deflect them back into traffic…Which makes me wonder…are there two sets of cables in that picture? It could be one for each side. Perhaps having only one set would allow the any car it catches to make it into oncoming traffic before it’s stopped.

Here’s some examples of what they are designed to do:
http://www.youtube.com/watch?v=6Wdbcwlu1gU&feature=related
http://www.youtube.com/watch?v=10L8421s18Q

The problem with this system is that sometimes they fail by allowing cars to get under them
http://www.youtube.com/watch?v=MCn6_Ouj64Q&feature=PlayList&p=202BF816769F4326&playnext_from=PL&playnext=1&index=28

[QUOTE]
[ul]
[li]Typically have a 4’ offset from ditch line due to potential drainage and maintenance concerns (wet medians, and conflicts with drainage structures)[/li][li]8’ offset with median widths greater than 60’ (greater the offset from the centerline of median to cable allows for improved mowing operations)[/ul][/li][/QUOTE]

http://www.ctre.iastate.edu/educweb/CE552/docs/TIG-CMB_Information_Module_Outline_-_030606.doc

I hadn’t thought about the center being too low. I can imagine a high speed car sailing over the top of it. From what I can tell (while whizzing by at 70mph), the cables are not attached to the posts at all. They are, however, firmly anchored at the ends. I’ve seen them after a crash and they seem quite effective at catching vehicles. As far as location-specific, I have seen them off center just about everywhere. I have also seen areas where there are two sets of barriers, one on each side. At this particular stretch, there is only one barrier on the westbound side. I travel on this highway quite often (was just there yesterday, in fact). This picture is a few years old, however. The outside breakdown lane has been remodeled on the westbound side of the highway. It is not a hurricane evacuation lane for people fleeing Houston and other coastal areas. The main change has been moving the rumble strip to the center of the lane and adding hurricane symbols every quarter mile.

Okay, that’s it. The SDMB is the most awesome website on the net. I asked what I thought was an obscure question and I got definitive answers within minutes. I even have research to back up the answer. I am even pleased that Texas seems to be doing well with implementation of this safety improvement. It isn’t often that I can be proud of Texas doing well at something.

I’m just curious about the cost. Early in the linked report, Texas claims to be installing 738 miles of cable barrier at a cost of $156 million. That’s a staggering $211,382 per mile. Ouch. However, later, they say the cost is closer to $65,000 per mile. Now that’s more like it.

I’ve heard these work great for most vehicles absorbing impact and all however I’ve also heard they are instant death for motorcyclists. Like getting tossed into barbed wire.

Minnesota has been installing them, and I wonder how they hold up to snowplowing?
The ones here are off center, just barely off the edge of one shoulder. I keep expecting to see 20 miles of cables and posts knocked over after a heavy snowfall, when 3 lanes worth of snow get shoved up against it.

If you consider the cost from proposal/tenders to completion, the higher price makes a bit more sense. The proper type of cable for the environment needs to be selected (I presume there are different ones for wet/snow regions and for dry/never see snow areas in terms of corrosion protection, longevity etc). A couple of companies may submit construction bids, and so consultants need to examine those bids and select the proper company for the job. Different companies may be doing different highways/counties as well for political reasons. Each mile of road has to be surveyed, studied and plotted in order to install the cables where they can be effective. Then they have to be installed, and a maintenance plan established and implemented. And you gotta pay the poor guy holding the “STOP” sign and holding up traffic while everyone else is getting all this done.

I have no idea what the going rate is for cable barriers, but it starts to seem more reasonable when you think about everything involved.

Well, it’s Texas, so I dunno if they get heavy snowfalls there. At least, I can’t remember anything like what we got here in Kansas last winter. Lemme tell you, it’s fun having to drive to work through that stuff.

Well, more proof that all one has to do is wait a bit and the Dope will provide. I had noticed this being done a few months ago on I-10 between Winnie and Beaumont, and was wondering why the barriers were offset as well. Thanks.

They’re just off the shoulder here in Tennessee too, at least on the one stretch of interstate I drive every day. I’ve seen a team of 3 guys with weed-eaters taking care of the grass around the cables and imagined how tedious it would be to weed-eat 20 miles of fence.

Does anyone know the cost for guardrails? I’m sure they’re more expensive but I really hate those ugly cables. Maybe I can start looking at them and thinking “I like that they’ve saved us money” instead of “How freaking ugly!”

The reason for a median barrier system to be located closer to one side or the other has to do with differing slopes. If there is a ditch in the median, the barrier is placed on the side that has a steeper slope to the ditch.

This makes sense when you think about it. You don’t want a vehicle to gain too much momentum before it hits the barrier from the steeper side. On the other hand, a vehicle coming from the less steep side will have to go up the steeper side of the slope so it will slow down more before hitting the barrier.

So the geometry of that highway’s cross section and median are likely different at various sections making one side steeper and then the other.

(my 2 cent WAG)
The center of the median is the lowest and where rain drainage collects and flows.
The ground here would be the softest most of the time so you probably don’t want to attempt to anchor something into that. A car hitting barrier posts that were stuck into moist ground would probably pull them right up.

Or kids on sport bikes trying to jump it. (warning: addictive video site).

Not the same thing, but in western Canada there was the case of a woman who hit the snowbank on a bridge and it acted as a ramp - she launched up and over the railing, 30 feet into the river below.

If it’s fresh fluffy snow it will stop a car. A bank of hardened ice after a week or so will probably launch anything lighter than an 18-wheeler.

I suspect the cables are easier on plowing than regular guard rails; the snow when plowed will easily go between the rails and is less likely to strain the whole setup. Of course, being far enough off the road, unless you get 3-foot snowfalls that should not matter.

There are a couple scenes in the linked Youtube videos above that clearly show the cables slicing off windows and gouging into the sides of cars. I’m envisioning some pretty nasty dismemberment if people hang their arms out of their windows, drive a soft-doored Jeep or if one of those cables snakes up and over the windshield of a convertible…

But it’s still better than flying into oncoming traffic and getting pulverized.