Car side and rear-view mirrors are designed to give you a view around your car, but they typically don't show you the two spots directly to the side of your car (the "blind spots"). Luckily, you can increase the field of view of your side mirrors by over 200%[1] and virtually eliminate blindspots by adding aftermarket blind spot mirrors.

The problem

Here's a photo of the view through my side mirror. Looks safe to merge left!

Photo of a side view mirror with no cars visible to the side

Oops! Turns out there's a car right next to me that I didn't see at all.

Photo out the driver's side window of the car showing a car directly to the side

The solution

With a 2" convex mirror attached to the side of my existing mirror, I can suddenly see everything in the blind spot.

Same photo of a side view mirror but with a blind spot mirror that shows a car visible to the side

Costs

Blind spot mirrors are incredibly cheap. I bought mine as a two pack at a gas station for $5 a few years ago, and you can buy them for less than $10 on Amazon.

Which ones specifically?

My blind spot mirrors are fixed with a black border. Being fixed in place makes them low maintenance, and I haven't touched them since they were installed years ago. I like the black border because it makes it easy to find them at a quick glance.

I can't find any that are exactly like mine on Amazon, although these borderless ones are similar. I suspect you'll find ones like mine at any sufficiently-large gas station.

I'm skeptical that adjustable mirrors will stay in place in 75 mph winds, but people on Amazon seem to really like these adjustable ones, and I plan to try them out on my fiancée's car.

Placement

You'll get the best field-of-view enhancement by placing the blind spot mirrors to the outer edge of your existing mirrors, and you should be careful not to cover any useful parts of the existing mirrors. I prefer to put mine in the outer top corners (like in my photos), but I've seen people suggest any corner of the existing mirrors that won't get in the way of the existing view.

Also, if you have an existing blind spot detection system (with lights or an alarm), make sure you don't cover anything it needs to work properly (I'm not very familiar with these, so I'd err on the side of caution and not add anything to your mirror unless you know it's safe to do so).

Mirror adjustment

You should also consider adjusting your side view mirrors to reduce blind spots on their own. You should adjust your mirrors at a minimum to they barely don't show the side of your own car (this is how I prefer them with a blind spot mirror), or go even further and make them only show your blind spot, which reduces the need for blind spot mirrors on sufficiently small cars.

A research caveat

While researching this post, I was able to find research showing that blind spot mirrors significantly increase your field of view[1:1], but I wasn't able to find any direct research on whether using these reduce the risk of crashes. This is very annoying, but I think it's likely that being able to see better will make your driving safer. Please send me an email or leave a comment if you happen to know of more direct research on this.

Final thoughts

The benefits of installing a blind spot mirror are clear: they're cost-effective, they drastically improve your field of view, and they simplify changing lanes. Although there's no direct research linking them to a reduced risk of crashes, the advantages they offer in visibility compared to their low cost make a compelling case for their use.


  1. Hassan, M., Tan, F., Abdullah, M., Radzuan, N., & Abu Kassim, K. (2020). Does a Circular Convex Blind Spot Mirror Increase the Driver’s Field of View?. Journal of the Society of Automotive Engineers Malaysia, 4(1). Retrieved from http://jsaem.saemalaysia.org.my/index.php/jsaem/article/view/117 ↩︎ ↩︎

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14 comments, sorted by Click to highlight new comments since: Today at 2:43 AM

I feel like this has come up before, but I'm not finding the post. You don't need the stick-on mirrors to eliminate the blind spot. I don't know why pointing side mirrors straight back is still so popular, but that's not the only way it's taught. I have since learned to set mine much wider.

This article explains the technique. (See the video.)

In a nutshell, while in the diver's seat, tilt your head to the left until it's almost touching your window, then from that perspective point it straight back so you can just see the side of your car. (You might need a similar adjustment for the passenger's side, but those are often already wide-angle.) Now from normal position, you can see your former "blind spot". When you need to see straight back in your side mirror (like when backing out), just tilt your head again. Remember that you also have a center mirror. You should be able to see passing cars in your center mirror, and then in your side mirror, then in your peripheral vision without ever turning your head or completely losing sight of them.

Is this the post you're looking for?

I've got a Mercedes with an Active Blind Spot Assist that eliminates the need to worry about this.

I was never able to get this working in a way that was fully satisfying, but I made a note to try it again. I personally like how I can look through one mirror and have all of the information I need to do a lane change instead of needing to look at multiple.

One big question I have here is about Chesterton's Fence. I don't understand why they aren't commonplace and it'd be, at minimum, helpful to understand the why.

Hopefully the answer is something like "the technology didn't exist however many years ago and the DMV is slow to adapt". But it also seems possible that they have researched it and found that it does more harm than good, eg. by adding more "visual noise" to the mirror.

I also think it's helpful to perform an inadequacy analysis here.

When I think about problems like these, I use what feels to me like a natural generalization of the economic idea of efficient markets. The goal is to predict what kinds of efficiency we should expect to exist in realms beyond the marketplace, and what we can deduce from simple observations. For lack of a better term, I will call this kind of thinking inadequacy analysis.

I'm not really sure what sort of efficiency I would expect here. On the one hand, government agencies are supposed to study this sort of stuff and add new safety standards when it makes sense. On the other hand, it seems plausible that they're just incompetent, poorly incentivized, and miss out on these sorts of low hanging fruits.

One thing to mention is that car manufacturers do hyper-optimize for official safety tests, so I consider the fact that any car manufacturers bother doing this even though it's not on the safety test to be a very big point in its favor, and the fact that most of them don't to be neutral.

So the mirrors only show the collision angles that are actually used in the officially approved safety tests?

Nice!

I don't think mirrors are considered in safety tests at all. My understanding is that the only requirement is that you meet the tech specs defined in Section 571.111 - Standard No. 111; Rearview mirrors.. These are all physical properties like the size, mounting and reflectivity. The regulations don't require cars to have additional convex mirrors, so I think it's unsurprising that car manufacturers mostly don't bother to provide them.

There are definitely cars where they're factory installed, like a lot of Ford vehicles and some models of various other cars. Also really large cars tend to have something similar, although they typically have an entire separate dedicated blind spot mirror.

I actually had the thought to buy one of these because a lot of rental cars have them (factory-installed) and I was annoyed that my own car didn't.

As far as I can tell, these are and have always been legal in the US (it's illegal for the main mirror to be convex, but adding an additional convex mirror is legal).

Newer cars are less likely to have these because they have fancier blind spot detection systems.

I find it confusing that they were never standard, but I get the impression that it's a mix of:

  • Adding more parts to a factory mirror costs more (especially at the higher-quality people expect from factory parts)
  • Buyers don't care (or the ones who do will buy a $5 stick-on)
  • Regulators don't care

I do find this confusing though, since it doesn't seem like anyone ever really argued that they're bad, but also car manufacturers mostly don't act like they're important. I wonder if it has to do with the lack of direct research on how they affect crash risk.

Ah that all makes sense. I didn't realize that some cars have them factory installed. That makes me feel very confident that the fact that their not commonplace isn't actually some sort of fence protecting against some unknown thing.

A couple of additional points, not yet mentioned, that might be helpful to those considering the problem and solution.

The small convex mirrors for showing the blind come in a variety of sizes and shapes. That should be consider when thinking about sticking one to your side mirror -- both for seeing the blind spot well and general side mirror visibility. I know I have found different ones to be more or less effective for me and I've used them on my race car where often you don't really have much time to look.

For those with some technical aptitudes cameras might be an even better solution. A fellow racer bought a rear facing mirror with a small display (would actual mount on the rear view mirror on a street car) for little over $100. The display actually allows two camera inputs and a split screen display. No reason those cameras could not be mounted in a position to show the sides of the car and are wide angel lens. The display unit could be easily located where the driver can see both that and still have focus on the direction of travel on the road.

Maybe I'm unusual in this regard, but peering at my mirrors closely enough to make out a car in one of these seems harder than just head-checking my blind spot?

Assuming that by head-checking you mean turning back: I don't like it because I'd lose eye contact with the cars in front of me. I typically move my head to the right until I see the blind spot in the left mirror.

I tried this a few years ago but ultimately I found something better: https://amzn.to/3PRMGN2 you can get mirrors that are also convex but go over the main rear-view mirror in your car. This was much more intuitive IME. (The link isn't the exact mirror I bought, I forget which one I got, but that general product there's probably a bunch)

(You should still test for yourself whether it allows you to see everything, of course)

I've been using blind spot mirrors for years and recommend them to everyone. At some point, I switched from circular to rectangular mirrors. One downside is that they're not very useful at night.