Tried and true - when speed data is needed, they do the job better than anything else.
Doppler radars are the oldest, but still most reliable method of gathering speed data in work zones. While the police have since moved away from K-band radar for the most part, for ITS applications, the accuracy and robustness of the technology is here to stay.
Dopplers are often overlooked by engineers designing specs around smart workzones. We'd like to see less of that, and for more people to treat dopplers as another kind of tool in the gathering of traffic data. They have a lot to offer, and have strengths the other more sophisticated sensors do not.
Doppler radar have been in use since World War II. The technology works by sending out a cone of waves from an emitter, and measuring the changes in frequency from the reflection of those waves with its receiver sensor. That might be an oversimplification, but it does the job of explaining how it works.
Dopplers strengths come from its simplicity. Theres no advanced configurations required - you simply power them up and point them at traffic - thats it.
Comparison vs Side-Fire Microwave sensors
Compared to side-fire microwave sensors, the difference is night and day.
A typical side-fire sensor deployment can take 15-45 minutes - a doppler on the other hand is deployed in less than 5 minutes. With side-fire sensors you need a laptop to plug in and go through its configuration process. Theres no such requirement for dopplers.
Through the life of a project, a side-fire microwave sensor might need to be reconfigured up to a half dozen times. Each time traffic is switched or shifted, the sensor needs to be recalibrate in order to maintain accuracy of its readings. Dopplers are much more forgiving. Sure, they need to aimed towards traffic, but due to their "cone", traffic can be shifted over a lane without any ill effect.
Most commercially available doppler radars are tested and rated to be accurate within +/- 1mph. Even the most well-configured side-fire can't reach that level of accuracy.
Any Vehicle Size
While side-fires do fine with normal cars and trucks, they lose accuracy when theres a lot of truck traffic. Dopplers don't have that limitation at all. Tractor-trailers, RVs, panel vans - a doppler can detect them with the same accuracy as a compact car.
Single Direction only
Since dopplers emit a cone shaped beam from them, they need to face oncoming traffic head-on. Due to this limitation, it is impossible for them to detect bi-drectional traffic as the side-fires are capable of doing. There have been some attempts to come up with a bi-directional radar, but so far I've yet to see one that can be deployed on a highway and maintain accuracy.
Dopplers are much more limited in the data they can gather. While side-fires are extremely data rich (speed, volume, occupancy, classification, etc), dopplers can only detect speed. Some dopplers purport to collect counts/volume, but their accuracy is limited and not fit for use in most ITS applications. The good news is, speed is the most important determinate of traffic conditions for the majority of projects.
Dopplers are much less power-hungry than side-fires. There are some very low power radars on the market from Houston Radar.
Dopplers need to be mounted so that they are secure and won't twist or otherwise have their angle towards traffic changed. As long as the doppler is facing traffic it should work fine.
You want them to be aimed at the flattest part of an oncoming vehicle, which would be the front grill. This means that you'll want to make sure the doppler is about waist height, but if the doppler has to be higher than that, say shoulder height because its mounted on a VMS, that is fine. There is a lot of flexibility here, but just keep it higher than about 3ft off the roadway. No mast required as it is with the side-fires.
Whats on the market
Theres only a couple of players in the US market. Stalker, Decatur, and Houston Radar. All three do the job, but one stands out as a the best in class, while still being competitively priced, and thats Houston Radar. They really have cornered the market on being the lowest power draw, which is important in solar/battery applications. Stalker and Decatur are more geared for police and sports applications, while Houston is focused on the ITS niche. With that focus, they really are the best choice in smart workzone applications.