In San Francisco, three right turns make a left
At many busy San Francisco junctions, turning left across oncoming cars is banned, so drivers circle the block with three right turns, a move locals call a San Francisco left. Taipei solves the same problem with its own detour signs. Such workarounds show how much of traffic comes down to managing who goes first.
Traffic covers people on foot and in vehicles moving over roads and other land routes. The word originally meant trade, as it still can, and came from Old Italian trafficare and Middle French trafique. Where the Italian came from is unknown; candidates range from a Catalan verb meaning to decant to Arabic words for distribution or seeking profit. It can describe air, rail, sea or network flows, though it usually means roads.
Organised traffic relies on lanes, priorities and control at junctions, and the United Nations' 1968 Vienna Convention on Road Traffic sets out the basic rules, though not every country signed it and local habits vary. The most fundamental choice is which side of the road to use. Whatever the rules say, drivers are still expected to avoid hitting anyone. Britain sets out its rules in the Highway Code, which mixes obligations with advice, while American states write their own codes, mostly based on the Uniform Vehicle Code. In Florida most traffic violations are minor offences separate from criminal law, whereas South Carolina has criminalised its traffic law, so driving five miles an hour over the limit counts as a misdemeanour.
Right of way decides who uses a contested bit of road first. Signs, lights and markings make it explicit, with the stop sign nearly universal, and a companion Vienna convention standardises such devices. Where nothing is marked, default rules differ between and even within countries. New Zealand, which drives on the left, required left-turning drivers to give way to oncoming traffic turning right from 1977 until it scrapped the rule on 25 March 2012. Without signals, one road at a busy crossing often dominates until a gap lets the other take over, and four cars meeting side-on at a crossroads can freeze in a jam.
Disruptions such as roadworks, collisions or debris can spread, and on busy freeways a small hiccup can linger as a traffic wave. Engineers model flows with queueing theory, stochastic processes and equations borrowed from mathematical physics.
Source: Traffic
