Technology has supplied our industry with a wide variety of traffic sensors. Each has its own set of advantages and disadvantages. Which one is best for your application? This may seem complicated at first, but the answers to a few simple questions will point you in the right direction.
First, please remember these are portable sensors. Ordinarily that means they should run off of 12 volt power, lending them to solar and/or battery power. They should have minimal power requirements, so they will run for an extended period of time without maintenance. And they should be easy to set-up and move around. Conditions on high way projects are constantly changing, so it must be easy to move the sensors to meet those changing conditions.
Next, think about the system to which these sensors will supply data. What data is needed? How often is it needed? How accurate must it be?
Queue warning systems just need speeds. So doplar radar works well and it is inexpensive. Doplar also works better at very low speeds. Microwave sensors provide additional data, but they are more expensive and don’t work as well below 20 MPH. Dynamic merge systems, like queue warning systems, just need average speeds. So doplar is best.
Travel time or delay time systems can be designed in a variety of ways. If most of the traffic continues on that route and doesn’t stop for any reason, probe data may work. Check the way it is reported in that location. They report speeds or travel times over a road segment. If that segment or segments match the route you want to watch, it should work well. However, if the segments don’t match up or if much of the traffic turns off or stops along the way, you will have to look at other options. One option is other forms of probe data. BlueTooth, toll tag or license plate readers can be placed at the exact limits of the route you are monitoring. But they are more expensive and still require a good volume of through traffic to report accurately. The easiest method uses spot sensors. In that case you gather average speeds at each location and calculate the average travel or delay time. Depending on the application, spot sensors are often more accurate that probe data
Will your work require you to relocate the sensors as work progresses? If so, that’s another good reason to use doplar sensors. Microwave sensors such as RTMS or Wavetronix, must be set-up and calibrated by a qualified technician. It doesn’t take long, but it is more expensive and must be scheduled. Doplar can often be relocated by laborers on the job. It is more forgiving in terms of aiming and distance from the live lanes.
On the other hand, some states now import portable sensor data directly into their real time traffic data. In that case, the portable sensor data should mirror the permanent sensor data as much as is practical. In those cases microwave sensors are probably the better choice.
The real lesson here is to consider what your needs are first, then find the tool that will best fill those needs.