Question on vehicle capabilties

[Mark_Pearce333]

HI

Does anyone know if there is a factor safety built into the swept path analysis? Like does the software only allow you to do, say 90%, of its capability? So that if you can get your route to work, it will definitely work in the real world.

Or does it do exactly 100% of capability with no room for error.

Hope this makes sense.

Cheers

Mark

[Tom_Frank]

Hi Mark,

My understanding is that there is no margin for error/safety built into the Vehicle Tracking software as default.

You will need to set these parameters yourself, the most obvious is to show offset margins around the body of the vehicle, altering the speed also affects turning movements. These are the two most obvious ways of factoring in a tolerance margin for safety and driver performance. Local highway can request these parameters to be changed from default.

The reason i don't believe there is any safety margin set as default is the following extract from the AutoTrack user manual, which is from when the software was owned and developed by Savoy. 

Limitations
Like any engineering design program, AutoTrack has limitations. In fact,
the list of factors that AutoTrack ignores is extensive and includes
dynamic effects, wind effects, acceleration and deceleration, sloping road
surfaces and slippery road surfaces.
Whilst these factors are by no means irrelevant, if we asked you to
supply a weather report, details of the condition of each tyre, a road
friction coefficient and full details of the suspension you would probably
give up and go back to using templates!
The fact is that AutoTrack is just like any other engineering program in
which you design for idealised situations and apply safety factors to allow
for real life variations.
This is exactly the way we recommend you use AutoTrack. Use the most
onerous vehicles likely in a particular situation and then allow a margin
for error on the results. The acceptable margin for error is something for
you to assess - if you are lucky it may be the subject of local design
guidelines.
Do not expect that just because a particular vehicle in the AutoTrack
library can perform a given manoeuvre on your desktop you will be able
to replicate it precisely in reality.
Do not assume that any of the AutoTrack defaults are recommendations
and if you have a reliable source of data, use it.
It all boils down to USE YOUR ENGINEERING JUDGEMENT

Last time I checked I couldn't find anything similar, or to the contrary in the AutoCAD Vehicle Tracking user manual, so we continue to apply our own safety margins.

Hope this helps.

[nigel_peters]

Like all the published design vehicles produced by DOT or highways agencies around the world AVT assumes perfect conditions and that the center point of the steered axle follows either a circular arc (ie none speed based turn), a spiral created by a set lock to lock distance (again none speed based) or a lock to lock time (speed dependent but the assumption is that the drive turns the wheel at uniform rate). 

The body then follows the movement of the steered axle based on the steering angle and the cut in point offset. Trailers do the same but base the movement of the towing units rear coupling instead of the steered axle.

Regardless of the method used to calculate the movement of the steered axle, the the spine has a transition which (in most vehicles and trailers) tends to a point when the whole vehicle is traveling in a perfect circle. From the dimensions of steered unit it is possible given any one for the following three values (wall to wall radius, curb to curb radius or the max steering angle) to calculate the other two.

Published design vehicles are 90 or 95 percentile of the dimensions of current vehicles of that type on the road.

Because of the assumptions used in swept path design and that they use standardized dimensions some governments like Australia require a safety margin of 300mm or more around the calculated envelope.

Regarding higher speed movements some countries state their design vehicles can only be used to model below 15kmh,  AASHTO require that the minimum turning circle being modeled for the maneuver is increased to take into account of the speed. AVT warns users if they model at higher speeds and supports the AASHTO equations and rules.

AVT will produce near identical (within mm's) turn templates to those published around the world, and also allows you to restrict the max steering, max allowed unit articulations and rate of turn to produce more conservative turns and also allows you to define two safety margins around the calculated turns.

Another common issue is most countries do not include wing mirrors on design vehicles and they can have a large impact on the end drive-ability of the design.

Good practice for swept path analysis is place the vehicle with sensible initial steering angles and articulation (ideally start on a longish straight section of road with the vehicle facing forward with zero steering), reduce allowed steering, articulation and lock to lock time to 90% or 95% of the allowed maximums, add a safety envelope over the results. Model using speed based steering and use a sensible speed for the situation and apply steering dynamics for speeds above 15kmh. Never use instantaneous changes in steering (turn on the spot) as it damages the road and/or increases tyre wear (I only ever suggest using turn on the spot for one off access to a construction site). Model the same maneuver for every type of vehicle expected to use the situation, and consider using larger cars than the standard as car sizes have increased a lot over the last decade. And finally for semi's consider defining two versions of the vehicle one with slightly different cutin point to reflect they have different cutin when fully loaded V unloaded (only New Zeeland have so far covered this issue in their design vehicles).