Here's such a system (on a truck, not a bike, and aimed forward) in action: http://www.youtube.com/watch?v=ridS396W2BY
Whatever is used and works, I still have trouble with seeing the utility. I car traveling at 40 mph is moving ~60 ft/sec and a bike at 15 mph is moving 22 ft/sec. Assuming it's coming at you with the intention of starting to move over to make at safe pass when 150' behind you, that's only 4 seconds. Lets say you'd only want a warning if the approaching car was within that, so less the 4 seconds warning. In that time you have to perceive and then react to the threat in some useful way. I don't see that as reasonable.
Use your ears and a $15.00 bar end mirror, YRMV.
As I recall, IR and acoustic currently don't really have the necessary range, at least not with what's reasonably affordable for something like this. The technology for laser ranging devices however is doable IMO (although I'm not 100% certain that you could have enough parallax for a direction). Honestly I think that image processing is where we'll eventually see this, combined with laser ranging or radar to provide scale. Given some sophistication you could determine an object's motion, even better if you had two or more video sources. The processing power is nearly there generally, if not already there with specialized processors.
This type of device could be put to good use (not simply an automated collision avoidance feature). We like mirrors after all, and mirrors at best show us what's approaching and just an idea of how fast. This device could do the same, but with more information having better precision and more consistently, and displayed (or otherwise interfaced) more conveniently. It just takes a little imagination.
If ever a thread deserved to come around again, I think this one was a good candidate. Personally, I'm interested in an affordable rear view camera that could accurately show what's coming with enough resolution to determine what lane they're in and how close to the fog line they are as they approach. If it worked with a smartphone or tiny tablet, that would be reasonable. I've seen one offered (I forget which webstore), but the reviews were horrible.
Another reason why this discussion could be made current is because Google just demonstrated their little self-driving pod-car, and it looks for pedestrians and, according to their blog, understands bicycle hand signals. Whether it understands the more common "point to where you want to go," I didn't gleam.
By the way, I have been buzzed (on US 1, a four-lane in FL), but I don't have a morbid fear about being hit from behind and understand it's the least likely cause to actually happen. It is, however, the aspect that I think my actions have the least effect on. I can minimize the risk of the other common scenarios by exercising common sense.
I just use a mirror. I'm reminded of the apocryphal story of how the US space program developed a special pen for zero gravity at a cost of millions of dollars and the Soviet program used pencils in space. Sometimes, old tech is best.
If you can't do great things, do small things in a great way. ~Napoleon Hill
Mirrors are light, waterproof, cheap, have no moving parts, do not require batteries, and never need winding. Most motor vehicles have at least 3 mirrors with most large trucks needing at least 4. You just need one itty-bitty one.
Sorry, I know this was not the answer you were looking for. But a mirror is THE answer for most cyclists with your concerns.
Last edited by JoeyBike; 06-08-14 at 09:24 AM.
"For all we know his skills may be excellent, allowing him to ride like an idiot without actually being one." - FBinNY
The company I drive for has VORAD or one of the equivalent on all the new tractors and I find it to be fairly useless. Mirrors are the way to go, warning devices are still too random and redundant for all but the very worst drivers who probably won't react correctly anyway.
Just to tinker, I was / am going to fabricate a rear sensor out of a radar unit that blind people attach to their canes that emits different tones when they are approaching stuff. My company makes them and I got hold of an out-of-date unit so just for fun, I was going to experiment with it.