Instruction manual

Appendix E. Road Tube Problems & Solutions

This appendix discusses the various problems that road tubes can cause. Road tubes present their own unique set of

problems for automatic vehicle classifiers & counters. Being aware of these before installing your road tubes can greatly

reduce the frequency of these errors.

This appendix divides each problem (or “error”) into a separate section, and then lists the known causes of the problem.

Appendix E.1 Missed Axles

Missed axles are the most frequent errors seen. They are caused, very simply, by the hardware (air switches or piezo)

inside the counter not reporting an actuation of the road tube when there is one. Some of the reasons for this are as

follows:

 Speed and Spacing.

The air switch in the Phoenix II is undoubtedly the best in the industry; however, even it is limited to 30

activations per second, or about 33ms per activation. If a second strike of the road tube occurs faster than

33ms, then the air switch will not report the second activation.

Does this ever happen? Yes, take for example a car towing a 2 axle trailer at 65 mph. A travel trailer typically

has a spacing of 2.5 ft between axles, and 65 mph is about 95ft per second. Therefore, how long does it take

between the first road tube hit by the travel trailer, and the second? This is (2.5ft/(95ft/sec)) = 26ms. The air

switch would not report the second axle of the travel trailer and this axle would be missed by the Phoenix II.

 Lifted Wheels.

Some trucks have an optional axle which may be raised slightly off the ground (to save tread ware). The

Phoenix II will not record it, but sometimes it can show up as an error if human observation data is being

compared to the counter and the observer is not aware that the wheel is lifted.

 Bouncing Vehicles.

Although uncommon, roads with dips or other irregular surface features can cause some truck axles to bounce

slightly. This can occasionally lead to missed axles. Note that the Phoenix II looks at both sets of road tube

activations, so this problem is minimized.

 Improper Road Tubes or Installation.

The type, length, and method of installation of your road tubes can lead to increasing the number of missed

axles; Always plug the end with a suitable device (unless the road tube is shorter than 25ft, and then DO NOT

plug it); always plug the road tube onto the counter nozzle all the way; always use an approved brand, size and

type of material for all of your road tubes; don’t overstretch the road tube because the diameter shrinks the more

you stretch it.

 Weak Signal with Longer Road Tubes.

Very simply, the longer the road tube, the farther the “sound” of an axle striking the road has to travel. Make

sure you use road tube lengths as recommended in the next section.

 Sound Wave Interface.

To understand why this is a problem, you should understand that the Phoenix II air switch (like all air switches)

uses a “sound wave” to detect an axle hit. This wave is very similar to a water wave, in that it starts at a point

and moves down the road tube to the round piezo disk sensor in the air switch. It travels down the road tube at

the speed of sound, which is about 767 MPH at 20 degrees Celsius, or 1125 feet per second. The force of this

“wave” of sound bends the piezo disk in the counter and causes a voltage spike to be generated. It is this

voltage spike which the Phoenix II detects as an axle strike on the road tube. The following example shows

how very close axle hits (such as with tandem axles on a truck) can actually interfere with each other and cause

a missed axle.

1) Assume you have a 50’ road tube stretched across a single lane of traffic. The road tube has been stretched

50” or 10% to make it tight. The end of the tube on the roadway is plugged and the other end is plugged into a

Phoenix II.

2) A 5 axle single trailer truck traveling 55mph crosses the tube.