
The first thing you notice in downtown Hanoi is the honking. More precisely, it's the zillions of scooters and motorbikes surrounding you, and their honking. Motorbike traffic outnumbers everything else on the road, with the possible exception of puzzled pedestrian tourists trying to figure out how to cross the street without getting run over (note the tire in the middle from the bike cutting through traffic -- BEEEEEEP). Motorbikes are also allowed places where cars are not, which means that there is no escape.
This turns out to be primarily a Hanoi phenomenon -- the motorbikes, not the honking. While motorbikes are prevalent all across Vietnam, they seemed to be most clearly dominant in Hanoi (as we went further south, the ratio of [motorbikes : (cars+bikes)] evened out somewhat). The honking, on the other hand, is a nationwide epidemic. At first, it was hard to figure out why people could possibly want to honk so much, and often for no apparent reason. And whether, with continual exposure to such levels of honking, there was anybody left with enough hearing to actually discern the various honks. Speaking of which, you'd be surprised at the variety of honking mechanisms found among your average Vietnamese vehicles: there's the regular single honk, the double-monotone honk (not to be confused with someone single-honking twice; these all require only one push of the thumb), the two-tone single honk, the two-tone multiple-honk, and the multi-tone melody honk -- think the General Lee from the Dukes of Hazzard.
After careful study of road traffic in four different cities, though, I believe I have discovered the secret of the honking, and really, driving in Vietnam in general: it is all a big video game.
The rules of the game are simple and a bit malleable, but the object is essentially to establish your sole domination of the road. Your primary tools are honking, passing, and careful lane positioning. Vehicles and pedestrians alike should bow to the will of your thumb, index finger, and acceleration pedal. However, as everyone else around you has the same objective, this is no easy task. The following seem to be rough guidelines for a winning strategy:
1. If you are a four (or more)-wheeled vehicle, at no time may all four wheels be in the same lane. Said event would be a clear demonstration of submission and unworthy pretending to the road-mastery crown.

2. If you are a two-wheeled vehicle, you should, like a small mammal backed into a corner, make yourself appear large and intimidating. This can be accomplished by (a) piling as many people and/or cargo -- boxes, hats, dozens of live pigs or ducks, your entire family -- onto your bicycle or motorbike as possible, and (b) weaving madly so that no one can see your true width. Also, since it will be hard for you to straddle a lane on two in-line wheels, you should attempt to occupy as much of a single lane as possible. Choose wisely -- the middle of the lane may not be the best place to accomplish this, as you leave room on both sides to be passed. Weaving thus becomes doubly useful. Honking, of course, replaces hissing and shrieking.
3.Since there are no double-yellow lines, passing may take place at any time, regardless of upcoming curves and oncoming traffic. The larger the vehicle passed, the more points. Bonus points are awarded when passing in traffic, hence you may expect to have to jockey for outer position when you have company. While attempting to block another driver's pass is sometimes successful, be aware that your opponent may score double points if executing a simultaneous-multiple-vehicle pass (that is, 2-vehicles wide, not consecutive vehicles). In such an event, your best strategy is to prolong your pass to such an extent that s/he is eventually forced to drop back in the face of an oncoming vehicle larger than him/herself.
The finer points of Driving: Vietnam may be explored later, perhaps with a quiz ... so stay on your toes!
(Also, as an aside, the book linked to in #2 is fantastic, and completely representative (i.e., not an embellishment) of sights in Hanoi.)