Fitting through tight spots in traffic is about learning to move your shoulders, your hips and bending at the knees. You should have a really good feel for how much space your body takes up and what you can do with it to slip through tight spots such as side-view mirrors between . Obviously that takes time. But this is what works for me best in these situations.
The widest parts of your body silhouette are your sholders and your handlebars. The widest parts of cars are their mirrors. Depending on how tall you are and whether the vehicle is a truck or a convertible. You will move different body parts to slip between them.
Fastest way to get skinny is to tuck your elbows in by straightening your arms. I use that sometimes to let pedestrians on the shoulder know that I'm not trying to hit them, its more of psychological message I send to them letting them know I don't want to hit them , and just by doing that they will also more likely get out of the way a little. Its a bit of a compromise, but all interactions with cars and ped are that way, you got to give a little space to get some space. But If I see the ped doesnt do anything make space for me as well, then in my head I say “fuck you were going to play chicken”. (I wouldn't recomend you do that though.)
Car mirrors are easy to avoid depending on their height by twisting you torso (without affecting your handling too much) to make a slimmer profile. This works when then mirrors are at hip to shoulder level for you. If the mirror is closer to your hip then to your shoulder you can also just stick your hip to the opposite side.
Now if the mirrors are at shoulder height or above then you can easily just drop of the seat and sit on your top tube by lowering your knee to lower your shoulders. This is much easier to do when you have a freewheel (another reason why I don't ride fixed)
The trickiest part of slipping through cars when the mirrors of two cars are adjacent to each other. They give you no space to work with...and if they are at hip height your fucked because you cant even duck down. The trick is to wait till are not adjacent and take advantage of that small time frame and twist your torso to get through. Be careful that the drivers don't slow down or speed up and their mirrors are adjacent again while your trying to slip through. It’s happened to me many times and what usually happens is that the mirror nudges me pretty hard until it flips inward. Usually after that point I apologize to the driver and flip the mirror out. By the way, I consider it very good form to flip a mirror back out of a car that you inadvertently flip in, its just good karma. Unless you flipped it in because the fucker was trying to sideswipe you.
Also one more thing you should be aware of how wide your profile is at pedal level. This helps in knowing when you can fit when your trying to sweep across to bumpers side/length wise without having your feet scrape the bumper.
Its almost like boxing. I have fairly wide shoulders but I can fit through almost anything because I know how to manipulate my shoulders/hips to slip and duck mirrors. I flipped in many mirrors in my time trying to gauge what I can and cant fit through. I still do ones in a while. You can only sharpen your body awareness through experience, the trick is to avoid being seriously hurt while doing so.
Just like in anything else in urban cycling, you will make mistakes, you will get hurt. The trick is in minimizing the damage so that you can learn from your mistakes and not be taken out of commision. You will make fatal errors when you dont keep a cool head and have some prior knowledge (which Im trying to give you) of whats going on. You will make mistakes, but keep them minor and deal with scrapes and bruises as opposed to to dealing with head injuries,broken collar bones and punctured lungs that come when you try to ride beyond your capacity. And yes you will know when your riding beyond your capacity when your terrified shitless, if you are...slow the fuck down and get out of the my way (and the cars) and let the pros do what they do.
BTW .Be careful of fenders on busses and trucks as they stick out more than the vehicle itself.
