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Curb Height Question

Question
State IL
Description Text

I work in the District One offices of the Illinois
Department of Transportation.

District One is comprised of the 6 County Chicagoland
Region.

The reason for my email is that the suburban bus company for
the region, Pace, is looking for permission to install 12-inch high curb along
approximately 20 routes in the Chicagoland that are under the jurisdiction of
the Department.

Having read through their literature it is clear that the
12-inch high curb has benefits to the transit provider however it is unclear to
me on its impacts to vehicular roadside safety.

Is 12-inch high curb TL1 crash worthy?

Will it increase the likelihood of vehicles flipping or
vaulting further when they leave the roadway?

I look forward to hearing back from you.

Keywords
Other Keywords Curbs
Date September 7, 2016


Response
Response

In general, moderate to tall curbs are not always desired along moderate to high speed roads due to some safety concerns. Of course, other benefits are provided by curbs, including pavement edge stiffening, road edge delineation, hydraulic drainage, etc. When curbs are struck by non-tracking vehicles, taller curbs can accentuate tendencies for increased roll angles, and possible rollover. Eventually and at some increased height, vehicle rollover tendencies go down. However, a tall curb would then be stopping and/or redirecting a vehicle if non-tracking, resulting in higher vehicle decelerations and changes in velocity, also possibly rapid yaw. At any rate, we do not currently perform crash tests under non-tracking impacts.

 

A 12-in. tall curb may not provide redirective capability for tracking impacts with higher center-of-mass vehicles. In a recent study performed in the early 1990s, we found that 12- to 14-in. tall timber curbs could only redirect a 2000P pickup truck at 15 mph and 15 degrees. This condition is lower than provided by Test Level 1 (31 mph and 25 degrees), which we deemed sub-TL-1 at the time. Later, we also developed two different curb-rail timber systems under TL-1 of NCHRP 350. For those systems, rail heights may have ranged from approximately 18 to 20 in. If desired, I could provide more information regarding these research efforts.

 

Date September 8, 2016


Response
Response

Attached is the research that was sent to me by the Chicagoland suburban transit provider, Pace, in order to justify the use of 12” high curb for certain transit stops along their system.

 

In my review of the literature it does not justify the use of the 12” high curb from a roadway safety perspective, but it does suggest that there might be some positives to vehicular traffic flow if the bus doesn’t have to stop & kneel at the transit stop.

 

Since I first emailed you the Department has agreed to allow Pace to implement the 12” high curb as a Pilot Project which means that no other routes along their system will be allowed to have 12” high curb until after their first route is implemented and monitored/studies for at least 5 years.  The roadway that the Department has agreed to allow as a Pilot Project is Milwaukee Ave from the Jefferson Park Transit Center in Chicago north to Golf Mill Shopping Center in Niles.  The roadway’s Posted Speed Limit varies from 30mph to 35mph and its ADT ranges from 20,000vpd to 30,000vpd.  The cross section of the roadway is 4-lane and goes from divided to undivided depending on the location.

 

Per what research I have done there is no curb height and design (e.g. curb face slope) that meets TL1 crash test worthiness while some 18” high curb designs do meet TL2 crash test worthiness.

 

That being said what you’ll find in the attached is that some entities, notably Grand Rapids, MI; Las Vegas, NV and Phoenix, AZ are placing the 12” or higher curb adjacent to the traveled way for transit accommodations.

 

The transit accommodations along Grand Rapids, MI (Silver Line) are rather new but have shown no adverse impacts per my discussions with them.  As such they are proposing similar transit stop accommodations/treatments along Lake Michigan Dr (MI 45) to/from Grand Valley State University and Grand Rapids. Due to the context of the surroundings along MI 45 vehicles should operate at a much higher speed than the roadways that the Silver Line runs along.

 

Another example is Cleveland’s Health Line (MAP) which has a similar treatment to Grand Rapid’s Silver Line.

 

This topic may be of interest to you and the MwRSF since, in my opinion, there is a big push to improve non-motorized facilities within this country potentially at the expense of the safety of motorized facilities.  In short the transit agencies want near level boarding, like a train stop, to improve their reliability.  With no research to prove the effects of higher than Department or AASHTO standard curbs this gives me pause from a liability perspective.

 

Date September 8, 2016
Attachment TR_PMO_MULT_StationPlatformHeightSafetyPrecedents (3).pdf


Response
Response

From your email below, it would appear that there is a recent trend or desire to use 12” tall curbs associated with transit operations, even along roads with moderate speeds. I guess my main thought may pertain to the increased risk of vehicle instability for passenger vehicle contacting tall curbs in tracking and non-tracking scenarios. If speeds are low enough, then I may agree that these elements are reasonable. However, the speeds noted below may still seem high enough to post those risks.

 

Non-cracking vehicle orientations may actually pose the greater risk as tripping could ensue upon impact. It would be interested to know whether any crash data has been analyzed for the recent sites with tall curbs to evaluate risk as compared to standard curb use. The risk of non-tracking vehicle impacts may be accentuated in locations where increased rain, snow, or ice may likely occur on road throughout the year. Grand Rapids, Michigan, should have those types of conditions more often than Las Vegas and Phoenix. Has anyone reviewed crash reports in these locations to actually see if tall curbs were struck? What percent of curb impacts resulted in rollovers?

 

I realize that only short segments of curbing would be mounted at 12” between the transitions to standard height. With short segments, the exposure is reduced somewhat. However, there still remains increased safety risk. In the end and in the absence of supporting study, I would agree that agencies could potentially be exposed to greater tort risk.

Date September 9, 2016


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