For the sake of putting a number to it you can think of "low speed" as speeds under 15 MPH, but that is just a rule of thumb -- there really is no specific speed where these rules all of sudden need to be followed, just keep these in mind for anything you may consider low speed.
The biggest factor that causes problems at lower speeds is actually a consistant speed (remaining at a speed while crossing the hoses). At lower speeds vehicles tend to change speeds much more quickly, especially in terms of the vehicles overall speed... a vehicle that increases speed from 5MPH to 6MPH is a 20% speed increase, whereas the difference of a vehicle changing from 50MPH to 51MPH is only a 2% speed increase. And at lower speeds the vehicle can more dramatically change speed in the few feet between tires, especially when slowing.
It is fairly common to want counts in many low speed situations. These are not the most ideal situation to collect accurate counts with hose-based counters, so you will have to take some special steps to get the most accurate counts. If you follow these guidelines you should be able to collect usable count data. Note: You cannot collect accurate classification data with a hose-based counter at low speeds, so attempting to collect vehicle classification data should be avoided in these situations.
For low speeds what you will want to do is just get the total vehicles passing by since the counter will have a tough time getting accurate classifications at those low speeds. Calculating the speed and spacings at lower speeds is a problem because vehicles can easily be slowing or speeding up at quite a high percentage of the speed they are traveling at, so the two sets of tires may be at quite different speeds when they cross the hoses, making axle spacing calculations inaccurate.
An additional problem at these lower speeds is that there will be a lot more "noise" on the hose (especially when you are down closer to 5mph), because the tires will be on the hose longer there will actually be more air pulses bouncing around in the hose.
So, to get the total vehicles passing at these low speeds, you will need to play with the "dwell" setting (one of only a handful of settings you will see after you download the data). You will keep both A and B dwells the same value. The dwell is the amount of time (in milliseconds) to ignore any further "hits" on a hose before it will start seeing hits again... at low speeds we recommend you set this up at 1000 (1 second) or higher... perhaps several seconds so you can be sure all axles (including any trailers) have also passed over. Some time that is shorter than the normal spacing between vehicles, but long enough for an entire vehicle to pass over the hoses. You would then edit your "Axle Correction Factors" to divide by 1 (instead of the normal divide by 2) since you will have just 1 hit for each vehicle.
In this scenario, you won't set your hoses out with any specific spacing, you would set one hose across one lane and the other hose across the other lane (if you have a median), or you would set it up as a short hose/long hose (a short hose across one lane, ending in the center of the road, and the longer hose across both lanes). Either of these two counts-only setups will give you directional (lane) data.
Finally, you will want to choose a location to set up your hoses where vehicles will not be stopping (including parking) on the hose. Apart from the obvious problem of passing vehicles not being seen due to another vehicles tires being on the hose, you may also have extra air pulses that bounce around in the tubes after the vehicle's tire leaves the tube (causing an additional hose hit to be recorded). This will cause a higher vehicle count than what actually occurred.
- You cannot collect accurate classification data at low speeds, attempt to collect counts only.
- Set dwells to a high value, one or more seconds (1000+ ms) in length, to attempt to get only one "hit" per vehicle.
- Set your "Axle Correct Factors" to a value of 1, to indicate there is 1 hit per vehicle.
- Avoid setting up the hoses in a location where vehicles will stop on the hose.