Raised Traffic Median/Curb Barrier - Urban Arterial

[HashimMG]

thread507-405695
Thanks to those who corresponded with the original thread.
The project was for a raised traffic median that dually served as the throat for a ventilation plenum of a subway below the street. The client was DOT but the agency for the subway was only going to allow it if the designer could justify that the structure would be strong enough that, paraphrasing: A truck/car or debris from the wall was not going to fall through the vent shaft upon a vehicle hit while a train was going by. It came down to whether the road was to be considered a Highway, or a street. As it was an Arterial and the matter regarded falling debris, Chap 13 seemed appropriate and we designed it to TL-3. That was later downgraded by agency to TL-2 on account of the posted speed limit (Ethics question). Any opinions on the approach taken?

 

[ACtrafficengr]

I'd go the other way and specify a bridge barrier. Your worst case scenario would be a truck or bus striking the barrier and either rolling over it or penetrating it. Bridge barriers are designed to reduce that risk.

Never use the speed limit to justify downgrading a barrier. Ask for the 85% speed under free-flow conditions. Even if the actual speeds do allow the lower barrier, I wouldn't use it unless either 1: the grating over the plenum will be rated for HS-25 loads, or 2: the plenum opening will be outside the deflection distance of the barrier. ADA-compliant grating should contain most small debris from the crash.

 

[Archie264]

>>>Ask for the 85% speed under free-flow conditions.<<<

ACtrafficengr, may I ask you about that parameter? I've heard of it but I have a hard time envisioning how it's determined. Is it a theoretical number based off of computer modeling or do they actually measure traffic speeds absent a speed limit?

I ask because it seems to me that measuring traffic speed in the presence of a posted speed limit would provide valuable information after the fact but I've heard that term used as justification for the value of the posted speed limit itself. And how could that be unless the road was allowed to exist for a while without any speed limit at all?

Asking to learn...

 

[BrianPetersen]

The 85th-percentile speed is supposed to be measured. For new construction, it may have to be estimated from measurements on nearby similar roads with similar circumstances.

Local, and sometimes not-so-local, governments and police departments are notorious for measuring actual traffic speeds and then using the finding that a huge percentage of the traffic is above the posted limit, as evidence that there is not enough enforcement, as opposed to what it really indicates - which is that the speed limit is too low.

I don't work in traffic engineering but this is a situation that irks me. In my province, outside of traffic-jam circumstances, 99% of traffic on 99% of roads exceeds the posted speed limit, frequently by a lot, and I fully admit to doing it myself. The municipality just north of me just spent a bunch of coin repaving and widening a section of rural two-lane road and adding bicycle lanes (which few will use - the road doesn't connect other frequent bicycle routes), and then changed the speed limit from an already-too-low 60 km/h to a completely absured 50 km/h. It probably should be 80 km/h. This was in conjunction with reducing the posted limit on all of their secondary roads previously posted at 80 km/h which no one followed, to 70 km/h.

Don't assume traffic will flow at the posted speed limit ... it may be ridiculous!

 

[Archie264]

Brian,

Yes, that's some of what I was indirectly wondering about/referencing. That is, it seems to me that the whole thing is a form of begging the question, that is, of assuming as true that which is to be proven. It seems to me it contains potential for mischief.