Axle counts to traffic volume

[corco]

Is there a quick and dirty factor one can use to convert axle count data into traffic volume data? (I assume there are more than 2 axles per average vehicle- I'd suspect the factor is somewhere between 2 and 3) This would be for an urban arterial

Thanks!

[mapman]

I don't know of a universal rule, because it can vary depending upon the percentage of truck traffic.  However, you're estimate of between 2 and 3 sounds correct to me.  If your arterial is a state highway, you may want to check with the state DOT to see what type of truck traffic it has.  Here in California, Caltrans publishes annual average daily truck counts (AADTs) for all state highways, broken down by number of axles.  Even if your arterial is not a state highway, you could use truck data from nearby state arterials as an estimate.  If all else fails, you could just assume the Highway Capacity Manual default of 2% trucks, assume that all of them are 3-axle trucks, and derive a factor from there.

[Chris]

I'm not sure how it works in the U.S., but here in the Netherlands, my axle counts are automatically divided by two, the so-called "axle pairs". Trucks however, have more than two axles, so you have to convert that. How much depends on the amount of heavy trucks (HT). This is usually around 2 % on minor roads, 5 % on major intercity roads and up to 8 - 10% on some non-urban freeways (usually major freight routes).

The total truck volume (LT and HT) is higher because there are also two-axle trucks, though these seem to be more rare in the U.S. because the cab-over layout isn't used as much as it is in Europe (we have a lot of 15,000 - 30,000 pound trucks driving around for city distribution).

Axle counts are outdated in my opinion. They can be used in situations where there is no option to do a classification count (axle distance or the total length of a vehicle and speed measurement), for example in places where there is almost always queuing traffic. I've also seen axle counts at parking lots, but those are rare, at least on this side of the pond.

[froggie]

Axle counts are outdated in my opinion.

Not necessarily.  Over on this side of the pond, axle counts are factored into ESALs (Equivalent Single Axle Loads) which are significant for their use in pavement design (type, thickness, etc etc).

[Alps]

Corco, short answer no.  You need truck percentage data for the particular road you're on.  As Chris says, at least divide by 2 to get axle pairs.  Now, what everyone else says about trucks is wrong.  Most trucks are 2-axle box trucks.  3-axle, 4-axle, 6+ axle trucks are all relatively rare and maybe add up to 1% of your total traffic.  5-axle trucks are your typical tractor-trailers, and those can be from 1% to 10% or more of your traffic.  Then you have hourly variation, weekend vs. weekday.  Weekday evenings you can easily get into the 60% range of trucks, in which case your conversion factor is quite a bit more than 3.  Weekend mornings you may be down around 2%-3% even on a major truck corridor.  (Note to any dissenters - I speak from a very detailed experience on this matter.)

[J N Winkler]

It is increasingly common for state DOTs to use WIM (weigh in motion) data to capture ESALs (Equivalent Single Axle Loads) without having to make assumptions about actual load in relation to GVWR (gross vehicle weight rating).  KDOT has used WIM data to estimate overweight percentages.  In combination with a design aim of providing indefinite pavement life for lengths of Interstate judged unlikely to need widening, that analysis was a factor in the decision to move to a standard 12" PCC (portland cement concrete) slab for reconstructed Interstates, in contradistinction to the 8" and 10" slabs used in first construction.

[corco]

Thanks! I decided that attempting to convert was a bit too complicated given the time crunch I was in and just presented the data as raw axle counts- that should be sufficient for the task at hand