Guidelines v Requirements

 

In relation to exit speeds from offset left and on-center designed roundabouts.=20

 

The reason for the exit speed being lower than the predicted speed at on-center roundabouts is simple. Well-designed on-center design roundabout have =

smaller R2 due to the smaller R3 than at an Off-set left roundabout which has a higher R2 due to the flat R3.=20

 

Also the predicted exit speeds are based on the formula in NCHRP 672 that u=

ses the 85th percentile acceleration rate. But that rate only applies to 85=

 percent of drivers. The worse drivers are the 15 percentile. They are the =

ones who typically

 cause the most problems, hit the curb on the exit exit, etc. These drivers=

 accelerate at a higher rate and hence achieve higher exit speeds. As the e=

xit radius becomes flatter the predicted exit speed increases dramatically =

compared to the significant limitation

 due to a =93small=94 R3 at an on-center roundabout. The benefit to pedestr=

ians is this =94small=94 R3 is significant.

 

 

=20

 

 

Michael Wallwork, PE

 

Alternate Street Design, P.A.

 

904 710-2150

---

Ken, I think you hit it on the head. The 85th percentile has a place, but i=

t's not in a residential street, commercial zone or any place with activity=

 that is not auto centric. When we as traffic engineers fall back to the ol=

d tried and true practices to defend our actions, we tend to forget that mo=

st were developed when the highway systems were being developed. Our cars w=

ere not as good, our technology in traffic control (signals) was just devel=

oping and our general lifestyle was slower. Today, we value choice of mode =

to a higher level, we realize the mistakes we made in the design of our tra=

nsportation networks. We , as an industry need to keep close attention to t=

he context of what we are trying to achieve. Are we designing or maintainin=

g a high speed limited access facility? If so, 85th percentile is as good a=

 metric as any for establishing sped limits and design standards. On the ot=

her hand, if we are building or maintaining a facility that will be shared =

with a variety of modes, design with a comfortable speed in mind. Use geome=

try and features to control speeds. Nothing says  "go fast" like a long str=

aight stretch of road with no traffic.

 

The other aspect of design lays at the feet of the planners. Why do we buil=

d schools on major arterials or busy collectors?   And then wonder why traf=

fic is too fast in school zones?  Many other examples as well. We need to d=

esign better.

 

Robert (Bob) Kurylko

Senior Transportation Engineer

Associate

 

Stantec

 

Phone: 204 928 8823

Cell: 204 918 2042

Fax: 204 284 4795

 

robert.kurylko@stantec.com<mailto:robert.kurylko@stantec.com>

---

From: Mark.Lenters@GHD.COM<mailto:Mark.Lenters@GHD.COM>

Subject: Re: Principles v Guidelines v Requirements

To: ROUNDABOUTS@LISTSERV.KSU.EDU<mailto:ROUNDABOUTS@LISTSERV.KSU.EDU>

There are some good points being made in the statements about the outlier d=

rivers, those speeding above the 85th percentile.  In traditional neighborh=

ood traffic calming the 85th percentile drivers were of concern but the com=

munity always remarked about those driving way above the 85th spot speeds. =

 I think that the perception of need in such cases is mostly based on the o=

utliers, so we frequently have a legitimate concern to attend to.  Thus, ti=

ghter exit trajectories can be in the toolbox of any aspiring designer and =

heuristics will confirm it in time.

 

There is a context for tighter exit design and most designers do appreciate=

 its applicability to single lane schemes.  However, it appears to be probl=

ematic for multilane exits due to path overlap issues and poor sight of the=

 exit crosswalk - vehicle to vehicle occlusion.  The driving task aspects o=

f curvilinear exits also demotes its use because the control task is priori=

 as the driver proceeds into the exit before sighting the pedestrian presen=

ce in a crosswalk.  Unfortunately, in spite of our technological advancemen=

ts, we are still serial processors when it comes to executing the driving t=

ask.  If not for the safety considerations multilane schemes the capacity e=

ffects of turbulence created when the exit 'valve' is made as tight as the =

entry 'valve' could be tolerated.

 

The gold standard for roundabout design remains to attend to the influence =

area of roundabouts (entry and exit) through gradual, transitional geometri=

c reduction of speed, creating slow entries that are comfortable and fluid,=

 but transitional to slow circulating speeds.  Then in the exits resumption=

 of travel speeds are likewise transitional and reasonably low where pedest=

rians are likely or expected.

 

No rules, just principles, so the variety of contexts receive tailor-made g=

eometry that promotes the selected balance and trade-offs between safety, c=

apacity and cost.  A road authority should be able to state the flows; a de=

sired entry and exit speeds (85th %ile); and, the design vehicle without ha=

ving to guide the designer further.  If context is agreed upon and thorough=

ly attended to, the results (performance) should be the same whether you or=

 I or anyone is driving the design.  This could be called performance-based=

 design specification.

 

Happy holidays,

 

Mark Lenters

Service Group Manager

GHD Inc. (The home of Ourston Roundabout Engineering)

---

Mark, Michael and all:

 

As you correctly point out Michael, the 85th percentile is a rubber

yardstick and one not to be used for determination of or reflective of a

specific safety gain.  It is OK, for example, to use the 85th percentile as

an indicator--such as in the case of So. Golden Road, Golden, CO where

between roundabout speeds were 33 mph versus the empirically measured

before signals.  The problem with applying the 85th percentile speed was

clearly outlined in research which found two "peaks" in speed--one for

older, mature driver and one for younger drivers. The 85th percentile melds

those two.  OK for horseshoes but not for determination of the safety--if

that is the goal--of exit speeds comparing two sets of geometrics.  Entry

and circulating speed "limits" as I understand it are fairly well defined

in terms of "maximum speed".  For example, Montpelier's 105 foot diameter

roundabout the maximum circulating speeds (tested by Dan Burden some years

ago) is about 22 mph.  Perhaps protocols (a matrix?) can be developed for

ease of application in varying R1/2/3 related to diameters/lanes.

 

Since the 85th percentile fails as a safety assessment tool, perhaps some

"further research" as Gene would say, is in order.

 

                           Tony

---