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Fast Exits

Page history last edited by Frank Broen 10 years, 3 months ago

> Subject: Re: no more fast exits in urban areas?

> To: ROUNDABOUTS@LISTSERV.KSU.EDU

>=20

> Michael, you may be correct. When Sweden starting building the left-offse=

t in the late 1980s, I brought up the point with potential high exit speeds=

 at pedestrian crosswalks (and at pedestrian non-marked crossings) but I do=

 not think the pedestrian crashes have materialized. Swedish roundabouts st=

ill have extremely low pedestrian crash rates compared to US intersections.

>=20

> An example of a well-built left-offset roundabout can be found at Google =

Maps (with street view) at Perstorp, Sweden at  N56.133128,E13.375425  . Th=

is is at the outskirts of an urban area with a pedestrian and bicycle path =

crossing Highway 21 on the east side of the roundabout and here is a comple=

tely tangential exit with 'no' R3 at all. But, thanks to the left offset, a=

nd a short R1, eastbound vehicles speeds are reduced from high speeds, post=

ed at 90 km/h (56 mph) until just before the roundabout in the in the eastb=

ound direction, where the speed limit drops to 50 km/h (31mph). I drove it =

this summer and R2 is around 60 feet and few people would drive it faster t=

han 16 or 17 mph, and --at least in Sweden-- most people do not accelerate =

that abruptly at the exit once their speed has been reduced. The most impor=

tant is to make sure that everybody (even aggressive drivers) gets well bel=

ow 20 mph in the roundabout, I think. (There is a nearby roundabout with ce=

ntered radial approaches that in reality has higher exit speeds.) I'd recom=

mend that people look at this design and some of the details. Since it is n=

ot in a downtown area with low speeds, there is no crosswalk at the pedestr=

ian/bicycle crossing. If there is anything unsafe with this design, it is r=

ather that westbound vehicles may be going too fast across the pedestrian c=

rossing prior to R1, and that the crossing is slightly too far from the yie=

ld line. Another interesting feature of the western approach here is that i=

t is a two-lane highway with continuous center cable barrier, one of the 's=

ecrets' to why Sweden has the lowest fatality rate per mile driven of any c=

ountry (with rural traffic).

>=20

> In conclusion, I think that left-offset should remain an acceptable alter=

native in real high-speed locations to ensure effective speed reduction for=

 the through movement.

>=20

> -Per

>=20

>=20

>=20

> Roundabout Research <ROUNDABOUTS@LISTSERV.KSU.EDU> writes:

> Ever since roundabout guide 1 came out I have spoken out against the offs=

et left design philosophy because of the high-speed exit which is a car ori=

entated design but it has mostly fallen on deaf ears because cars are still=

 more important than people. Studies done by the Minnesota DOT and UNC that=

 have shown low driver yield rates to eds as exceeds increase have been ign=

ored by off set left advocates. Interestingly when faced with this data the=

y ignore or have no answer.

>=20

> I suggest you all read page 6-69 of NCHRP 672, copy below you will note t=

hat a disadvantage f off set is the lack of exit speed control and accelera=

tion through the departure crosswalk. Despite this clear statement so many =

engineers ignore it and its impact on pedestrians. Conversely an advantage =

of the Radial design is it can control exit speeds. Maybe engineers get it =

so wrong because in the disadvantage of the radial design NCHRP states that=

 the increased (should be "decreased") exit radius reduces (should be incre=

ases) control of exit speeds. Note that radial designs have decreased small=

 exit radii and therefore how can a smaller radius increase exit speeds ove=

r a tangential exit radius??

>=20

> With NCHRP, the corrected version, stating radial designs control vehicle=

 speeds and hence acceleration through crosswalks why do engineers ignore t=

heir own guidelines?

 


 

<mjwallwork@ME.COM<mailto:mjwallwork@ME.COM>> wrote:

 

My point is not not necessarily about pedestrian crashes but pedestrian delay, pedestrians having to wait upwards of a minute or so until a gap or until a driver yields. Pedestrian mobility is about delay, convenience as much as crashes.

 

Sent from my iPad

 

On Dec 7, 2013, at 2:06 PM, Gene Russell Sr <geno@KSU.EDU<mailto:geno@KSU.EDU>> wrote:

 

To say otherwise is to defy the laws of physics. I am not against low speed  in urban areas where we have pedestrians; however i am not convinced it cannot be achieved with left offset. I'd like to see  more more raised crosswalks and MORE ENFORCEMENT AND HIGHER FINES ( yes,another soapbox of mine)  Also, from a human factors standpoint, it takes some driver concentration to negotiate curves. That's why curves anywhere are high crash risk road sections. That's why I wonder if a driver with a straight ahead view of the crosswalk will be more cognizant of a pedestrian in an exit crosswalk than a driver paying attention to negotiating a sharp curve on exit,     (another broken record i know) but we should have some driver simulator research on this.  Gene

 

________________________________

The offset left does not necessarily produce a high speed exit.  The exit speed is a function of the entry speed, circulating speed, and acceleration out the exit.

See Baranowski and Waddell, 2004:

http://www.nc-ite.org/images/files/Baranowskipaperfinal.pdf

 

________________________________

Date: Sat, 7 Dec 2013 16:45:21 -0500

From: Per_Garder@UMIT.MAINE.EDU<mailto:Per_Garder@UMIT.MAINE.EDU>

Subject: Re: no more fast exits in urban areas?

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

 

Michael, you may be correct. When Sweden starting building the left-offset in the late 1980s, I brought up the point with potential high exit speeds at pedestrian crosswalks (and at pedestrian non-marked crossings) but I do not think the pedestrian crashes have materialized. Swedish roundabouts still have extremely low pedestrian crash rates compared to US intersections.

 

An example of a well-built left-offset roundabout can be found at Google Maps (with street view) at Perstorp, Sweden at  N56.133128,E13.375425  . This is at the outskirts of an urban area with a pedestrian and bicycle path crossing Highway 21 on the east side of the roundabout and here is a completely tangential exit with 'no' R3 at all. But, thanks to the left offset, and a short R1, eastbound vehicles speeds are reduced from high speeds, posted at 90 km/h (56 mph) until just before the roundabout in the in the eastbound direction, where the speed limit drops to 50 km/h (31mph). I drove it this summer and R2 is around 60 feet and few people would drive it faster than 16 or 17 mph, and --at least in Sweden-- most people do not accelerate that abruptly at the exit once their speed has been reduced. The most important is to make sure that everybody (even aggressive drivers) gets well below 20 mph in the roundabout, I think. (There is a nearby roundabout with centered radial ap!

 proaches that in reality has higher exit speeds.) I'd recommend that people look at this design and some of the details. Since it is not in a downtown area with low speeds, there is no crosswalk at the pedestrian/bicycle crossing. If there is anything unsafe with this design, it is rather that westbound vehicles may be going too fast across the pedestrian crossing prior to R1, and that the crossing is slightly too far from the yield line. Another interesting feature of the western approach here is that it is a two-lane highway with continuous center cable barrier, one of the 'secrets' to why Sweden has the lowest fatality rate per mile driven of any country (with rural traffic).

 

In conclusion, I think that left-offset should remain an acceptable alternative in real high-speed locations to ensure effective speed reduction for the through movement.

 

-Per

 

 

 

Roundabout Research <ROUNDABOUTS@LISTSERV.KSU.EDU<mailto:ROUNDABOUTS@LISTSERV.KSU.EDU>> writes:

Ever since roundabout guide 1 came out I have spoken out against the offset left design philosophy because of the high-speed exit which is a car orientated design but it has mostly fallen on deaf ears because cars are still more important than people. Studies done by the Minnesota DOT and UNC that have shown low driver yield rates to eds as exceeds increase have been ignored by off set left advocates. Interestingly when faced with this data they ignore or have no answer.

 

I suggest you all read page 6-69 of NCHRP 672, copy below you will note that a disadvantage f off set is the lack of exit speed control and acceleration through the departure crosswalk. Despite this clear statement so many engineers ignore it and its impact on pedestrians. Conversely an advantage of the Radial design is it can control exit speeds. Maybe engineers get it so wrong because in the disadvantage of the radial design NCHRP states that the increased (should be "decreased") exit radius reduces (should be increases) control of exit speeds. Note that radial designs have decreased small exit radii and therefore how can a smaller radius increase exit speeds over a tangential exit radius??

 

With NCHRP, the corrected version, stating radial designs control vehicle speeds and hence acceleration through crosswalks why do engineers ignore their own guidelines?

 

 

 

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Per GÃ¥rder, Ph.D., P.E., Professor

Department of Civil and Environmental Engineering

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------------------------------

 

Date:    Sun, 8 Dec 2013 13:06:23 -0500

From:    Per Garder <Per_Garder@UMIT.MAINE.EDU>

Subject: Re: no more fast exits in urban areas?

 

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Gene, I agree that it should be studied. But better than simulator studies may be to look at pedestrian crash records. And, if we have hundreds of a design with no crash in many years, then safety must be okay. Though as Michael points out, delay is

another important aspect of pedestrian quality of service, -Per

 

 

Roundabout Research <ROUNDABOUTS@LISTSERV.KSU.EDU> writes:

>That's why I wonder if a driver with a straight ahead view of the crosswalk will be more cognizant of a pedestrian in an exit crosswalk than a driver paying attention to negotiating a sharp curve on exit,     (another broken record i know) but

>we should have some driver simulator research on this.  Gene

 

 

 

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