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AutoTurn

Page history last edited by Frank Broen 10 years, 11 months ago

PART 1

 

Date:    Tue, 9 Apr 2013 17:57:57 +0000

From:    Milton Carrasco <mc@TRANSOFTSOLUTIONS.COM>

Subject: AutoTURN and AASHTO Design Vehicles

 

ALL

 

There has been some discussion about AutoTURN, its use or misunderstanding  on its accuracy etc.  It is not possible to fully respond to these comments /statements without writing a mini thesis, however we hope that this short  and the next couple of responses will clear some of the air around these is sues.

 

For the purposes of understanding key factors and components of the AutoTUR N software, I have broken the write up into three parts:

 

Part 1 - AutoTURN Accuracy and AASHTO Design Vehicles Part 2 - Factors affecting turn simulations Part 3 - A guide on how to use AutoTURN in your projects

 

I have been prompted to do this by others in the group as a 'educational' c ommunication.  Personally, we see the same type of misunderstanding across  a very broad cross-section of our user base.  This primarily comes because  the software is easy to use that people do not self-educate themselves on t he underlying principles!  I welcome your feedback, as we will also pass th ese scripts on to our wider user base.

 

 

PART 1 - AutoTURN Accuracy and AASHTO Design Vehicles

 

The AASHTO design vehicles, that are included in the AutoTURN software, are  based on data provided in the Green Book.  In AutoTURN these vehicles are  not editable as they are intended to represent the standards provided and t o produce consistent results.

 

 

  1.  AutoTURN Accuracy:

     *   Transoft has conducted several field studies using articulated buses and tractor semi-trailers operating at low speeds (under 10 mph) to veri fy the accuracy of the algorithm used in AutoTURN

     *   The results have shown that the algorithm, which is based on the 'constant pursuit method' of analysis and is kinematic in nature, produces a ccuracies to within 3% of the swept path at its widest point in our field t ests.  This difference is on the conservative side, meaning that the softwa re produces a 3% wider swept path, which we believe is plenty accurate for  road design purposes.  In our field tests on an articulated bus, the maximu m swept path was 5.0m (16.4ft) in width, where as the software produced a w idth of 5.15m (16.9ft).

     *   Even at higher speeds of up to 40mph, the differences between the software and the field measured paths are only slightly larger.  These stud ies were competed using GPS receivers mounted on the tractor and semi-trail ers units.

     *   Even recent studies based on maneuvers of complex wind blade trailers conducted in Colorado in 2012, have shown insignificant differences bet ween field measurements and AutoTURN output.

     *   Hence for a given centerline turn radius, AutoTURN produces accurate swept paths for any specified vehicle, whose turn limitations have been  specified correctly.

 

 

 

  1.  AASHTO Design Vehicles:

     *   NCHRP 505 Report provides the basis for the 2011 AASHTO Green Book

 Design Vehicle specifications.  Page 22 of that Report notes that "A design vehicle is not intended to represent an average or typical vehicle in its  class but, rather, to have larger physical dimensions and a larger turning  radius than most in its class."  Hence it can be said that most vehicles i n a specific design vehicle class could and will likely be able to make a t ighter turn than the specified design vehicles of that same class.

     *   NCHRP 505 Report (Table C-1) indicates that the steering angles used in the 2011 Green Book are for the most part based on those used in the 

2001 Green Book.  Two major issues need to be pointed out here:

                                                               i.      It is not known what research was done to determine the steering angles for the se vehicles in preparation for the 2001 Green Book.

                                                             ii.      The NCHRP 505 Report recommended that the 2001 Green Book steering and tractor-t railer angles be adopted for the 2011 Green Book.  This means that if the d ata in the 2001 Green Book was incorrect or outdated to begin with, then th e data in the 2011 Green Book could be several years out of date.

 

     *   The decision on the AASHTO design vehicle turning specifications lies solely with AASHTO (and appears to be based entirely on the recommendat ion of the NCHRP 505 Report)

 

 

To conclude, AutoTURN is proven to produce accurate simulations.  It can be assumed that, based on the 2011 AASHTO design vehicle specifications, the  resulting AutoTURN simulations of these design vehicles should be accurate.

  The AASHTO design vehicles specifications were developed by researchers as part of the NCHRP 505 Report.  As noted in the NCHRP 505 Report, design v ehicles are intended to represent the largest vehicle in that class.

 

Further, the 2011 AASHTO design vehicles are based on the turning characteristics contained in the 2001 Green Book.  There is a good possibility that  these turning specifications are out of date, and hence considerably over e

stimate the 'design vehicle's turning capability'.   So if the AASHTO desig

n vehicle turning specifications are overly conservative, the AutoTURN simulations will also be overly conservative (or vice versa).

 

While AutoTURN can model any vehicle provided the user inputs the required specifications, designers require to be cautious in doing so, unless the da ta is proven to be representative of that class of vehicle.

 

I will publish Part 2 in the next few days..... give you all time to digest and comment on Part 1


Comments 

Sent: Wednesday, April 10, 2013 2:35 PM

To: ROUNDABOUTS@LISTSERV.KSU.EDU

Subject: Re: AutoTURN and AASHTO Design Vehicles

 

G'day Milton

 

Thank you for your explanation I really appreciated it.

 

It is good to know that AutoTURN represents the largest vehicles in its class and that AutoTURN is slightly conservative as many of us users tend to believe. However, it is of concern that with the changing vehicles characteristics working in tighter environments that AASHTO may be out of date in respect to steering angles and lock to lock time. I see some of the modern buses with power steering do amazing things at roundabouts with lock-to-lock times in seconds, well below the six seconds in AutoTURN.

 

Hopefully, AASHTO will see its way to updating it data base.

 

I hope AutoTURN retains the WB-50 that AASHTO deleted. WIth tighter and tighter site designs and narrow residential street designs with small radii I think theWB-50 will be around a long time, maybe not on long haul but certainly within town deliveries.

 

Another issue is that many vehicles manuals my clients receive do not have or have in them a readily accessible location detailing the steering angle or lock to lock information making field tests necessary.

 

One issue that Transoft could help with is articulated trucks that so often make what is called a "square turn". It would be nice if AutoTURN could make our life simpler by including a special template for "Square turn" to help designers represent real-world driver behavior. At the moment I am guessing by using a tight steering angle. An accurate method of drawing real world "square turns" by articulated vehicles would be welcomed. Maybe Transoft could add this feature. Thanks.

 

Michael Wallwork, PE

Alternate Street Design, P.A.

904 710-2150

 

From:    Ken Sides <Ken.Sides@MYCLEARWATER.COM>

Subject: AutoTURN and AASHTO Design Vehicles

 

To Michael Wallwork's offering of suggestions to AutoTURN I'd like to add some pointers from my own (painful) experience:

 

1) Be aware that the graphic depicting vehicles is "dumb;" that is, it is not linked to the computer software.  This matters because the fire engine (or maybe bucket trucks, too) may be depicted as having a ladder/bucket hanging over, say, the REAR of the truck, but YOUR truck ladder hangs over the FRONT.  So watch out for that AutoTurn" gotcha."

 

2) It seems most fire trucks and solid waste trucks are ordered custom.  That is, the purchaser specifies the chassis they want and a mix & match of various things that bolt on to it.  For this reason, you likely won't find your vehicle already in the AutoTURN library. 

 

3) You can and should try to get the various AutoTURN input parameters from the manufacturer of the vehicle, but you may have to go make your own vehicle measurements.  (For max fun, this task is best carried out on the hottest, most humid day of the year.  Expect to do this at the end of the drivers' shift, when they are dying to go home but instead are staying late to drive their trucks in circles just for you.)  

 

3) When measuring the turn radius for solid waste trucks you will encounter a big surprise:  when the truck completes the circle, it's not a circle!  It's a widening spiral!  That's because those big tires scrub when making a full-lock turn.  This also explains why the truck driver has had an amused look on his face the past hour.

 

4) I have AutoTURN plots in hand for the Fire and Solid Waste departments for these 3 reasons:

 

a) To confirm that the roundabout can do what I want.

 

b) To reassure the FD or Solid Waste departments that I have, indeed, listened carefully, taken their concerns seriously and cranked THEIR exact, specific vehicles into the design.

 

c) To provide graphic documentation to those departments, if asked, that their needs are taken care of. 

 

5) The AutoTURN plots are also useful to demonstrate that the outrigger equipment (ladders, buckets, rotating sweeper brooms, etc.) will not collide with signs, trees, etc. that are close to back of curb. 

 

-Ken

Ken Sides, PE


 

 

 

 

 

 

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