I've been shopping eyeglasses which include mirrors so you can see behind yourself.
Why? I'm tired of bicyclists sweeping up behind me on the sidewalk, at speed, six inches from my body. I do sidewalk-scans better than anyone I know, and still got caught three times last month. People entering the sidewalk from a doorway or car behind me and running ahead also aren't safe. Using rearview mirrors reduces the cost of scanning for new traffic behind.
What types are available? I've done a lot of web searching, and still am not sure. Testing notes on four options are available below. Aside from bicycle mirrors attaching to the eyeframe, or small mirrors in the corners of the glasses, the main idea seems to be to apply a layer of mirroring on the inside lens of the glasses, on the outside third or so. It'd be great if this was available as a standard option on any set of glasses.
How well does it work? Very well. Takes a little training, and then you have to grow new sidewalk-scanning patterns, have to make it a habit. But it has already saved my butt a few times. During a quick walk outside today, I realized I now felt naked without it.
Angles: If you wear them down on your nose, or up above your ears, the mirrors will be pointing at the sky. When they're down on the ears and up on the nose, and your head is straight, you'll be looking right behind you, just by shifting your eyes to the lower-right or lower-left. Makes me conscious of walking with my head straight. Takes a little adjustment going up and down hills to be able to see the sidewalk behind.
Lighting: Another surprise is how much the external light affects visibility. It's easier to see in the mirrors if the rear is brightly lit and you're walking into shade. But if you're walking into the sun, then it's hard to see in the mirrors. A brimmed hat can make it darker behind the lens, easier to see into the mirror.
Reality: Bicycle forums have had long flamewars about whether mirrors make you more safe or less safe. The big argument seems to be "It doesn't replace a headturn when changing lanes", and that's true. But it does reduce the cost of checking behind, and should be supplemented by full headturns as needed. Sometimes you need to pay attention to tricky areas being traversed... sometimes you can't risk the balance issue from a quick turn and return... sometimes oncoming traffic needs attention and you shouldn't risk blindness behind... sometimes a headturn actually sends a signal to other people in the traffic stream. A quick discreet check of the mirrors is much easier.
- A clip-on-eyeglasses bicycle mirror costs $10-15... looks dorky, works great. Even though it's on only one side, it's easy to move the head and scan the entire surrounding area.
- $5 plastic sunglasses actually work well. They seem to be made in Taiwan, and are available at novelty and spy shops. But they're big and plastic and make me look like El Rayo-X.
- There are hardshell oval lens glasses, each 2.5x1.5 inches, thin metal frames. Cost is usually $20-30, sometimes coming with a case. This is the one I've been using the most, even though it's a little dark, and the mirrors don't seem quite as sharp as some of the others.
- There are also more rectangular lenses, also $20-30, about 2.25x1.125... look almost like pince-nez, but a little thicker, and with earpieces. These are the darkest I've used so far, but with a very sharp mirror image. I think they'll work if regularly moved up and down on the nose... otherwise, it can be hard to see in the shade. I'll probably bring these as a backup to Taiwan.
A novel danger: Be careful in traffic when starting. That mirrored stripe means that sometimes there will be a flash which appears to be in front of you and off to the side. It's startling to be standing at a corner and to see cars suddenly rush by where they shouldn't be. I still flinch at the false signal sometimes, but it will become natural with practice.
Practice: Try tracking an item as it moves behind you... a pedestrian you've passed, someone standing by their car, someone walking at an angle behind. Try switching mirrors while watching something behind. Check weird headturn angles, trying to catch sight of the object at one try.
What'll I do? Well, if someone comes up behind me, right down the center and unsafely, I'll step aside. If there's no room for them to pass, I'll stop and block them with the cane, with a "what are you doing" mien. But preferably I'll see them 20-30 feet ahead, and just be able to do the standard cane-twirl warning, no confrontation and no eye-contact, but the moving object makes them go "uh-oh", very naturally. (From my usual immediate reactions after nearly getting hit on the sidewalk I was first afraid that I would use the hook of the cane on their neck, or the tip to kill their spokes -- it's that upsetting to be at the mercy of an idiot -- but once I tried the glasses I realized I had many more options, and could deter an assault rather than just punish it.)
Buying tips: Try searching with terms like "('rear view' OR 'look behind') sunglasses mirrored"... lots of garbage results, but some real places. Amazon has a few, through third-party sellers. Distribution doesn't seem mainstream... there's lots of amateur storefronts listed that I wouldn't trust with my purchase, not sure how secure their databases are.
Additional notes: Doesn't work that well for girl-watching, although it's still better than nothing. ;-) On the bus it can be weird. It's much much better for estimating speed of an overtaking pedestrian that repeated headturns are. I wish there was lighter-colored glass and more frame options.
Bottom line: Having discrete mirrors in sunglasses while walking makes it easier and cheaper to frequently scan the entire 360-degree range of potential traffic. It takes a little training to use this additional capability effectively, and then to make it habitual. But, having used it, it's hard to consider going back to irregular headturns as the sole way to protect from idiots behind.