Thursday, September 23, 2010

Detention time verses dwell time

Recently, I was asked to explain the difference between dwell time and detention time.  This is a common question that plagues the transportation industry and as such there is likely more than one explanation.  Here is how I explained it.


The first question that some of you will ask is why is this even a discussion?  The answer in the transportation industry is that you generally only get paid when assets are being utilized.  The challenge becomes knowing what has moved to make money and what has just moved in general.
For the purposes of this discussion the following definitions are required for the reader to continue.
Detention Time
detention Time graphic
This is the calculation that something has actually physically not moved.  This uses the specific latitude and longitude reading of a GPS device and checks to see if the asset actually moved.  This can be very helpful to find assets that are “static” regardless of where they are.  An example of this is a dropped trailer that has been forgotten or that has been abandoned by a tractor. 
1) See what is actually not moving, literally.
2) Find misplaced or static assets regardless of where they are suppose to be.
The biggest challenge is to account for GPS drift.  GPS drift is when the latitude and longitude can shift due to GPS tracking precision.  GPS location is based on triangulation.  As the number of satellites change over time the accuracy of the latitude and longitude can vary.  This is called drift.  The 4 decimal place precision is roughly 11 meters.  What this means is that over time a very static GPS location device could look like it moved up to 11 meters and it did not actually move.  This is addressed by using math to account for the drift and then comparing the results against a drift algorithm.  The answer will come back with it did or did not move + or – of the drift value.

Dwell Time

Dwell time graphic
Dwell time time is very similar in thinking to detention but with one large variance.  Dwell time is primarily focused on assets that have NOT left a geo-fenced area.
A geo-fenced area is when you place a boundary around a specific landmark.  This landmark could be a terminal yard, customer site or any other geo-spacial location that has a complete closed logical perimeter (circle, square or polygon).
Dwell time will tell us if an asset has NOT left a defined location.
One of the biggest benefits to the organization is that it will tell you if an asset has not left a location.  What can cause this is if the asset has been shunted around the yard for various reason but has not left the yard.  Customers will do this with trailers specifically.  They could be using them for mobile warehouses yet from a latitude and longitude perspective the asset is moving and would not be caught by using latitude and longitude only.
The biggest challenge facing dwell time is the interval of the GPS latitude and longitude reporting.  Let’s say that you have a device that is reporting location every 8 hours.  It is possible for a pulse to happen within the location, then, the asset leaves the location and returns prior to the next pulse.  This condition would give a false positive.  To combat this you may have to look at both detention and dwell to isolate patterns.  Another alternative is to increase the polling cycle.

Business questions answered

1) What is not moving = Detention
2) What is not moving and I do not care where it is = Detention
3) What is hanging around a yard and not making money = Dwell
4) What is at my customers and not moving or leaving = Dwell
5) Looking for lost revenue for assets domiciled at customer locations = Dwell
6) Misplaced assets = Detention
7) Dropped trailers = Detention

Visit my trailer tracking site to learn more.

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