question: why do buses make that "PFSSSST" sound when they stop?
comment: Many large vehicles have a system of brakes actuated by compressed air instead of hydraulic fluid or cables you would find in a lighter vehicle or passenger car. Parking/emergency brakes on such vehicles are usually "normally closed": to release the brake, air pressure must be applied to the brake. For these, a sudden release of air pressure will quickly activate that brake, but also will create a little bit of noise.
passages: BULLET::::- August 2014: A bus driver was fired for "screaming" at a teenage boy after he accidentally pressed the "stop button" too early by accident. The bus driver was said to have suddenly stopped the bus and forced the boy off the bus on an isolated stretch of road in Perth's southeastern suburbs.
   Vehicle Sound for Pedestrians or VSP is a Nissan-developed warning sound system in electric vehicles. The Nissan Leaf was the first car manufactured by Nissan to include VSP, and the electric car includes one sound for forward motion and another for reverse. The VSP was also used in the Nissan Fuga hybrid launched in 2011. The system developed makes a noise easy to hear for pedestrians to be aware of the vehicle approaching, but the warning sounds do not distract the car occupants inside. Nissan explained that during the development of the sound they studied behavioural research of the visually impaired and worked with cognitive and acoustic psychologists, including the National Federation of the Blind, the Detroit Institute of Ophthalmology, experts from the Vanderbilt University Medical Center and a Hollywood sound design studio.
   BULLET::::- Minimal noise pollution - Unlike trams or gasoline and diesel buses, trolleybuses are almost silent, lacking the noise of an engine or wheels on rails. Most noise comes from auxiliary systems such as power steering pumps and air conditioning. Early trolleybuses without these systems were even quieter and, in the UK at least, were often referred to as the "Silent Service". This however can also be seen as a disadvantage, with some pedestrians falling victim to what was known as "Silent Death" (in Britain) or "Whispering Death" (in Australia).
   "Vroom" is cited in early childhood language development texts as an example of a common early word or sound made by young children in the course of play. Anthony D. Pellegrini writes in the chapter "Interpreting Children's Play" in the book "The Puzzling Child: From Recognition to Treatment" (1982):
  
  The social phonological play of two year olds is exemplified by use of conventional noises, through syllable repetition, to identify actions, events, and objects...Two children may be playing parallel or associatively with trucks. Their phonological play would be the "vroom" sound of truck engines
   Nissan's Vehicle Sound for Pedestrians is a sine-wave sound system that sweeps from 2.5 kHz at the high end to a low of 600 Hz, a range that is easily audible across age groups. Depending on the speed and whether the Leaf is accelerating or decelerating, the sound system will make sweeping, high-low sounds. For example, when the Leaf is started the sound will be louder, and when the car is in reverse, the system will generate an intermittent sound. The sound system ceases operation when the Nissan Leaf reaches and engages again as the car slows to under . For the 2011 Leaf, the driver could turn off sounds temporarily through a switch inside the vehicle, but the system automatically reset to "On" at the next ignition cycle. The system is controlled through a computer and synthesizer in the dash panel, and the sound is delivered through a speaker in the front driver's side wheel well. Nissan said that there were six or seven finalist sounds, and that sound testing included driving cars emitting various sounds past testers standing on street corners, who indicated when they first heard the approaching car. Nissan removed the ability to disable the pedestrian alert between model year 2011 and 2012 in anticipation of the U.S. ruling to be issued by the National Highway Traffic Safety Administration.
   The mechanism by which a PTU works is by surging, PTUs self-start by pure mechanical influence alone resulting from the delta-pressure between the two hydraulic systems it is connected to. Consequently, a PTU accelerates very rapidly under the delta-P induced load, and then stops just as suddenly once the pressure equalizes. Each pressure surge may only be a second long, causing a stop-start mode of operation.
  
  In practice, this results in a 'whoosh-whoosh' sudden spool up and spool down, which produces a loud noise than can be likened to a barking dog.
   The primary duty holder under the Bus Safety Act is the operator of the bus service, as the person who has effective responsibility and control over the whole operation. However, the Act also contains a safety duty covering "people with responsibility for bus stops", including people who design, build, or maintain the stop, plus those who decide on its location.
   A pivoting arm equipped with a stop sign is a piece of equipment required by law on North American school buses. The sign normally stows flat on the left side of the bus, and is deployed by the driver while picking up or dropping off passengers. Some buses have two such stop arms, one near the front facing forwards, and one near the rear facing backwards. The stop sign is retroreflective and equipped either with red blinking lights above and below the legend or with a legend that is illuminated by LEDs. Unlike a normal stop sign, this sign requires other vehicles travelling in both directions to remain stopped until the sign is retracted.
   This duty was introduced in response to research showing that the most serious hazard associated with bus travel occurs when passengers, especially children, are crossing the road after alighting from the bus. The location and layout of a bus stop is therefore a factor in the level of risk.
  
  Safety duties are also imposed by the Bus Safety Act on a range of other people including -
  
  BULLET::::- "bus safety workers" including drivers, schedulers who set bus timetables, and mechanics and testers who repair or assess vehicle safety
   Buses and trains are supposed to have either recorded announcements or driver announcements of station stops, but these announcements are sometimes muffled, inaudible, or omitted by automated systems. In the event that automated systems are not functioning properly, the vehicle driver or conductor is to announce stops over the public address system.
  
  Section::::Hearing impaired.
  
  The MBTA has a TTY number for "T" information: (617) 222-5146. Many stations have TTY pay phones; the MBTA web site has a list.
   BULLET::::- 1990: When MARTA, Atlanta's rapid transit rail system, extended its train service to the Hartsfield International Airport, the band marched single file back and forth through the length of the train, playing "Leaving on a Jet Plane" and "Off We Go into the Wild Blue Yonder".
  
  Section::::References.
  
  BULLET::::- Article in Spoleto Today covering the band's appearance at the 2009 Piccolo Spoleto arts festival - May 24, 2009
  
  BULLET::::- Public Broadcasting Atlanta's television show "This Is Atlanta" featuring the band in 2008 - Originally aired July, 2008
   The bus stop "flag" (a panel usually projecting from the top of a bus stop pole) will sometimes contain the route numbers of all the buses calling at the stop, optionally distinguishing frequent, infrequent, 24-hour, and night services. The flag may also show the logo of the dominant bus operator, or the logo of a local transit authority with responsibility for bus services in the area. Additional information may include an unambiguous, unique name for the stop, and the direction/common destination of most calling routes.
   Section::::Sign variants.
  
  Although all English-speaking and many other countries use the word on stop signs, some jurisdictions use an equivalent word in their primary language instead of or in addition to it. Israel uses the image of a hand in a "stop" gesture.
  
  In some Caribbean and South American countries (Argentina, Bolivia, Brazil, Chile, Colombia, Cuba, Dominican Republic, Ecuador, Paraguay, Peru, Puerto Rico, Uruguay, and Venezuela), signs bear the legend ("stop" in Portuguese and Spanish). Mexico and Central American countries bear the legend ("halt") instead.
   The developing process took between nine months to a year, and three sound companies sent in synthesised WAV file samples that were evaluated by Fisker employees and executives. The prospective sounds were studied in an audio chamber to allow engineers to evaluate the sounds without other noise interfering. After testing the candidate sounds in different locations relative to the vehicle, Fisker fine-tuned the final sound with its own equipment. The warning sound is activated when the car is travelling at less than .
  
  Section::::Specific systems.:Ford.
   The research was conducted by the Transport Research Laboratory, and the findings were published in 2011. The study found little correlation between pedestrian vehicle involvement density and noise level for the majority of vehicles. In addition, the analysis found no evidence of a pattern in pedestrian vehicle involvement densities when only considering those accidents occurring on or slower roads, or where the pedestrian was disabled. A previous study did not find an increased pedestrian vehicle involvement density for electric and hybrid vehicles with respect to their conventional counterparts, which raised the question as to whether added sound is necessarily required. The study also noted that some modern conventional cars are as quiet as their electric counterparts, even at low speeds.
   Most bus stops are identified with a metal sign attached to a pole or light standard. Some stops are plastic strips strapped on to poles and others involve a sign attached to a bus shelter. The signs are often identified with a picture of a bus and/or with the words "bus stop" (or similar in non-English-speaking places).
   Some transit buses, such as those in New York, have had, since at least the 1950s, turn signals activated by floor-mounted momentary-contact footswitches on the floor near the driver's left foot (on left-hand drive buses). The foot-activated signals allow bus drivers to keep both hands on the steering wheel while watching the road and scanning for passengers as they approach a bus stop. New York City Transit bus drivers, among others, are trained to step continuously on the right directional switch while servicing a bus stop, to signal other road users they are intentionally dwelling at the stop, allowing following buses to skip that stop. This method of signalling requires no special arrangements for self-cancellation or passing.
   Section::::Vehicular device types.
  
  Section::::Vehicular device types.:Acoustic.
   Section::::Regulations.
  
  Since 2009 the Japanese government, the U.S. Congress and the European Commission are exploring legislation to establish a minimum level of sound for plug-in electric and hybrid electric vehicles when operating in electric mode, so that blind people and other pedestrians and cyclists can hear them coming and detect from which direction they are approaching. Tests have shown that vehicles operating in electric mode can be particularly hard to hear below .
  
  Section::::Regulations.:European Union.
   BULLET::::- Mr. Junkett: Junkitt is an African-American Vehicle Maintenance Inspector in the episode "Revving Up." As a minor antagonist, he accidentally leaves a peanut butter sandwich on the bus's carburetor (blocking air inflow), which then drips peanut butter into the bus's fuel line and cylinder's spark plug, which makes the bus break down. Voiced by Sherman Hemsley.
  
  BULLET::::- Mr. McClean: Voiced by Malcolm McDowell. The school janitor, who struggles to control the new automatic welcome system in "Gets Programmed".
   Upon boarding the bus, for example, route 240 serving Edgware, iBus plays the announcement "240 to Edgware" and corresponding text appears on the visual displays As the bus approaches the stop, the on-board system will announce and display the bus stop name. Since March 2014, the space below this name shows the current time. When a passenger has requested the bus to stop, this is replaced by the message "Bus stopping".
   Electric vehicle warning sounds are sounds designed to alert pedestrians to the presence of electric drive vehicles such as hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), and all-electric vehicles (EVs) travelling at low speeds. Warning sound devices were deemed necessary by some government regulators because vehicles operating in all-electric mode produce less noise than traditional combustion engine vehicles and can make it more difficult for pedestrians, the blind, cyclists, and others, to be aware of their presence. Warning sounds may be driver triggered (as in a horn but less urgent) or automatic at low speeds; in type, they vary from clearly artificial (beeps, chimes) to those that mimic engine sounds and those of tires moving over gravel.
   Trolleybuses are quieter than internal combustion engine vehicles. Mainly a benefit, it also provides much less warning of a trolleybus's approach. A speaker attached to the front of the vehicle can raise the noise to a desired "safe" level. This noise can be directed to pedestrians in front of the vehicle, as opposed to motor noise which typically comes from the rear of a bus and is more noticeable to bystanders than to pedestrians.
   APS in other countries may consist of a short recorded message, as in Scotland, Hong Kong, Singapore and some parts of Canada (moderate to large urban centres). In Japan, various electronic melodies are played, often of traditional melancholic folk songs such as "Tōryanse" or "Sakura". In Croatia, Denmark and Sweden, beeps (or clicks) with long intervals in-between signifying "don't walk" mode and beeps with very short intervals signifying "walk" mode.
  
  Section::::Enhancements for disabled people.:Relief symbol.
   For operational purposes, there are three main kinds of stops: Scheduled stops, at which the bus should stop irrespective of demand; request stops (or flag stop), at which the vehicle will stop only on request; and hail and ride stops, at which a vehicle will stop anywhere along the designated section of road on request.
  