Ion Speed Pro. Good value, around $50-$75.
It's shaped pretty much like rumrunn6's Polaroid -- I think that model Polaroid is manufactured by Ion to Polaroid's specs.
Weatherproof as-is, no special housing needed, as long as the rear cap with O-ring is in place. It's been through several downpours, no problems.
Runs 90 minutes per charge. To run longer I remove the weatherproof rear cap and plug in a USB cable to a battery. This compromises the weatherproof integrity, so I can do this only in good weather.
Saves to mp4. Each file is about 40 minutes, with a seamless transition between files (ideally -- there are occasional quirks).
The ultra-wide lens with 180 degree FOV isn't optimal for traffic documentation. It's intended to be a close range action cam. For traffic documentation I'd prefer around 120 degree FOV. At 180 FOV with a 5mp resolution, it's difficult to make out license plate details even in daylight unless I'm right on the vehicle's bumper.
Keep in mind that many sample videos on YouTube demonstrating traffic documentation cams are taken in Europe, where license plates are larger and easier to record. It's much more difficult to record US plates, especially in Texas where the black on white lettering is excessively contrasty, especially at night.
Daylight image quality is very good -- snappy color and contrast without excess. Auto-exposure shifts rapidly without fluttering, to accommodate moving between light and shadow. Low light is fair, nighttime is mediocre. At night it has high noise, poor dynamic range, and cannot reliably record white reflective Texas license plates. For nighttime use, at best it will record the basic shape and color of the vehicle and just enough details to help with identification. So far the only altercation I've reported was a Domino's delivery driver who hooked me on a left turn at an intersection at night. The video had enough detail to identify the driver. (I only asked the safety rep to reinforce courtesy training, not to fire the driver, since there was no physical or verbal conflict, just rude driving. That incident occurred in an area with lots of cyclists.)
It will also record still photos. I typically fall back on time lapse mode after the battery gets low enough that it won't record video. I set time lapse to every 5 seconds, the smallest available interval. The intervalometer offers every 10 seconds and slower/longer increments, which I've used at home a few times to create time lapse videos of passing storms, etc.
I rigged up a homebrewed mount using the supplied car mount shoe, to get it lower on the helmet. Used heavy duty plastic Velcro tape and zip ties to secure it, along with the supplied safety leash retainer.
On the road bike I use the supplied plastic mount that clamps onto the stem, handlebar, etc.
Eventually I plan to add a rear facing camera.