Traffic Calming is the practice (particularly prevalent in the UK) of erecting physical barriers to attempt to slow traffic down within residential areas.  Different tactics include:

	*Speed bumps or "sleeping policemen" - rounded humps in the road that give you an unpleasant jolt if you drive over them too fast.  Sometimes responsible for the removal of loose exhausts, and (allegedly) the occasional oil sump.
	* Speed ditches - basically a giant line of a pothole that you can't drive around.  These are especially nasty because unlike speed bumps (which are often painted for visibility) they sneak up on you.  At high enough speeds a speed ditch can flatten your tires.
	*Roundabouts - see TrafficCircle.  In the UK, these are used more for managing traffic flow than for TrafficCalming, since they don't do a great deal to reduce speed. ''In fact, these increase the mean speed, compared to (more familiar to north americans) intersections.''
	*Odd gate things that narrow the width of the road so that you have to drive slowly, lest you lose your door mirrors.
	*Giving way to oncoming traffic.  In places, the road is artificially narrowed (by putting in kerbs) such that only one car can pass.  In order to proceed, you have to stop behind the kerb and give way to oncoming traffic.  Bonus points are given for alternating the priorities at successive choke points.
	*Sharp corners on narrow roads, instead of wide corners on wide roads as is typical in suburbia.
	*Stop signs.
	*Rumble strips. These are thin strips on the road (about an inch high, i'd guess), usually painted yellow, which come in flocks, usually just before a junction where the strip-bearing road gives way (eg the stem of a T-junction). They are named for their purpose: they make a low rumbling noise as you drive over them. Cunningly, the strips get closer together as they go along, so in order to maintain a constant pitch to the rumble, you have to slow down; your brain apparently wants a constant pitch, so you slow down automatically. Indeed, if you hold a constant speed, it feels like you're speeding up!
*Slaloms (aka, Pretty obstructions and diversions ... in your face).  Here in Mountain View and in Berkeley (California), finding that your road suddenly terminates ''in a giant flower pot'' (with usually quite attractive plants growing in them) or concrete sculpture of some kind isn't out of the ordinary.  The trick is to navigate around them, in a kind of 1/4-traffic circle.  That is, to go straight, you need to make a left-turn around the statue, then a right-turn onto the street you ''were'' on prior to the flower pot.  It works great -- and is ''#(*$&#ing'' annoying.

Ironically, these measures calm the traffic, but don't necessarily calm the driver.  They've also been blamed for putting excessive wear-and-tear on vehicles (e.g. speed bumps are quite hard on the shock absorbers). 

----

Traffic-calming measures have been criticised for increasing the response times of emergency vehicles.

There appears to have been at least one study on emergency response times - a report on that said "A study was done by the city of Portland, Oregon, to determine the validity of these claims.  The study found that, regardless of the emergency vehicle driver, the increased response time was on the order of 0.0 seconds (for 22-ft. speed bumps) to 10.7 seconds (for traffic circles), much less than the minutes of additional time originally conceived. "

The study is 

Atkins, Crysttal and Michael Coleman. "The Influence of Traffic Calming on Emergency Response Times." ITE Journal August 1997: 42-46.

Referenced in http://www.ic.arizona.edu/~ce467/ResourcesHandouts/GroupLiteratureReviews.doc which contains a useful summary of other studies 

-------

Arguably, they encourage people to drive as fast as possible between the obstructions, in order to save the time lost by slowing down at the obstructions. ''This is the kind of criticism that can only be considered with hard data, though.''

It's been argued in some communities that narrowing the roads increases the risk to cyclists. Of course, that's not the case when you're narrowing the roads for the express purpose of providing dedicated bike lanes (eg, Paris). And it could never be a factor when bike lanes are separated from car lanes by a physical obstruction such as a kerb (eg, Netherlands). See http://www.io.com/~bumper/ada0002.htm

They've also been accused of redirecting driver attention on the obstructions and distract from pedestrians and other vehicles.  There appears to be no hard evidence for this, though.

The main complaint about traffic calming seems to be from people who complain it slows *them* down instead of those other idiots who really need it.

Also known (in some circles) as TrafficTormenting.  

-----

[ Please attempt to avoid ThreadMode on this page as much as is possible, thanks ]

''Speed ditch''

Never seen one around here. Where do those appear, then?

Define 'here' -- 'here' in California, they're seen a lot in southern California (e.g., Oceanside, Vista, and even some parts of San Diego).

----
The authorities in the "northern and western suburbs of Washington, D.C." (otherwise known as Montgomery County, Maryland) have discovered TrafficCalming.  They are renovating streets to make them less driveable and slow down traffic.  The irony is that Montgomery County is already one of the great gridlock capitals of the world (gridlock = congestion, traffic jams), yet the authorities are trying to ''slow down'' traffic.  Are they insane, evil, or what?

I ''like'' the roundabouts when traffic is moving -- they make far more sense than 4-way stops -- but when congestion sets in, the roundabout fills up with blocked traffic and nobody can move in ''any'' direction.  So naturally they install them in places where traffic is guaranteed to back up into the roundabout.

----
See also:
	* http://www.trafficcalming.org/
	* http://www.ardue.org.uk/world/roadrage.htm