Side-fire Sensors - General Info

Today we're starting out with the big dogs in the ITS world. Side-fire microwave sensors. This will be a quick breakdown of whats available in the market, some pros and cons of each, and some general guidelines to follow when setting them up. A future article will go into greater detail on each, but for now, lets take a look at the overall market. My qualifications to write this article can be summed up quite simply: I've deployed and maintained over 500 (yes, five hundred) side fire radars across the US for use in traffic studies, DOT data supplementation, and of course smart work zones. I've also trained dozens of traffic control companies, DOTs, and other agencies on their usage and maintenance.

Wavetronix vs G4 vs Speedlane

For about a decade there were only two real players in the market. The Wavetronix Smartsensor, and the RTMS by Image Sensing Systems. They've each had several iterations, Wavetronix came out with the "HD" version, and ISS has the "G4" (and now the SX-300), but the core product is the same. In the last year theres a new contender on the market, the Speedlane by Houston Radar.

The basics.

All three sensors operate in the same way. They sit on the side of the road, approximately 20ft from the edge line, perpendicular to the traffic. They go up on a mast of some sort, and cast a beam across the road. A good analogy is, its like a garage door beam. If you cut the beam, the door stops coming down. These sensors cast a beam (or two as we'll see below) and as cars go through this beam, the sensor records traffic information. Because the sensor beam crosses the road at a 90 degree angle, these sensors have the HUGE advantage of being able to detect both directions of traffic.

The information gathered by these sensors is pretty rich, it includes:

  • Traffic speed
  • Traffic Volume (aka counts)
  • Roadway Occupancy Percentage
  • Gap/headway (the distance from vehicle to vehicle)
  • Vehicle Classification (length of vehicle and car vs truck vs semi)

All this data is in stark contrast to the other type of popular sensor used on smart work zones, the much heralded doppler radar. While dopplers only gather speeds (and in some cases a rough estimation of volume), these side fires gather all the data a traffic nerd could want.


Here are some general pros and cons of the sensors as a whole - this is not model/brand specific.


Bi-directional traffic detection
TONS of data
Remote configuration is an option
Camera option provides (a somewhat limited) "Eye in the sky"
Tend to be the preferred non-intrusive detection source for DOTs


Require a laptop to setup
Require reconfiguration if theres a traffic switch
Expensive compared to dopplers
Speed/Volume readings can be skewed by heavy truck traffic due to occlusion
Requires a 20-30ft mast
Some brands of sensors are power hogs, requiring more solar/batteries


There are a number of things to consider when side-fire radars are spec'd into your project. The biggest is what sensor are they looking for. Depending on the wording of the spec, the agency might be gunning for one particular sensor over another. If the spec mentioned "Dual Beam" anywhere in it - they are going for the wavetronix. If the spec just talks about "being able to detect up to 8/12/16" lanes of traffic" and theres no mention of "dual beam" anywhere - you're free to use the RTMS G4 without any issue.

The other big consideration is truck traffic. If you are deploying on a major trucking road, these sensors are not necessarily the best suited to handle the job. These sensors have trouble detecting traffic on the other side of trucks, due to whats called "occlusion" as seen here:

Not only that, but they have trouble accurately determining the speed and volume of semi-trucks. Sometimes the trailers show up as separate vehicles, and when truck traffic makes up a large percentage of the total vehicles, that can really skew your counts. There are techniques to avoid these issues, specific to each manufacturers sensor. In general though, if you can avoid using side-fire sensors on major trucking routes, do so.


Once you've determined the need for a side-fire radar, the next important consideration is the platform. What is going to be sitting on the side of the road for the length of project? It has to have adequate solar and battery to power your sensor (and likely modem), and it also has to have a mast high enough so that the beam cast by the sensor is higher than the majority of traffic. There are a number of sensor trailers on the market, or if you have solar/battery platforms available you can modify something that will work. I've seen arrow boards retrofitted by adding telespar to their cage, adding telespar to PVMS, and all kinds of home-brewed solutions work. As long as it powers the device 24/7/365, and the mast is reasonably sturdy, you should be good to go.

Wavetronix vs G4 vs Speedlane
The Wavetronix Smartsensor

The Wavetronix products are widely considered as the "Cadillac" of the side-fire radar world. While they accomplish the same thing in the end (bidirectional detection - lots of metrics), the Wavetronix have two beams that shoot across traffic. With two beams, their able to reliably and accurately measure traffic very well. These units are robust, I've personally had very little problems with them, and they're easy to setup by showing you each cars speed in their configuration menu. The setup is also made easy by the lanes auto-defining themselves within the sensor (though you still have to plug a laptop into the sensor to verify things). They also have an arrow that pops up on the screen while configuring, that shows you if the sensor is properly aligned or not. While they are nice to setup and maintain, they are also the most expensive option on the market.

Image Sensing System's RTMS

Image Sensing Systems (ISS) has been in the side-fire radar market for at least a decade. They've gone through many revisions, starting with the X2s, X3s, G4,s and their latest, the SX-300. All these sensors share the same generally - single beam detector in a compact package for an affordable price. They meet the same specs as the Wavetronix, particularly when "dual beam" isn't mentioned specifically. Even when it is, I've seen cases where the RTMS is similar enough to fit the bill. The configuration of the sensor is very much the same as the Wavetronix, but in the case of the RTMS you run a "Wizard" type setup process, where they walk you through most of the steps needed. The RTMS typically have more options available to them over the wavetronix, including a basic pinhole camera, and a new bluetooth traffic sensor on their latest model.

Houston Radar SpeedLane

The newest contender on the market, from my personal favorite start up in Texas, Houston Radar is called the Speed Lane. Debuted at the 2015 National ATSSA show, this sensor is hot off the presses. Like the wavetronix, it is also a dual-beam sensor but combines a fixed camera and bluetooth local connectivity (so you don't have to get out of your truck to configure it or pull data). The setup for the Speedlane is similar to the Wavetronix, where you see traffic (with each vehicles speed) on a detection "map" but instead of auto-defining the lanes, you just click your mouse a couple of times and define them yourself. While the product is new to the market, Houston Radar have long since proved themselves to be an excellent manufacturer of traffic detection devices. I suspect that given a little time, this sensor will prove to be very competitive with the old guard incumbents.

Wrap Up

As you can see, this is a lot of information to go over. In future articles, I will post some cheat sheets on how to setup and configure each of these sensors and go into the specifics of overcoming deployment challenges. This should give you a good foundational knowledge though about how the sensors work and some things to consider when choosing a sensor for your next project.