question: Why isn’t possible for all cars to move in unison when a red light turns green instead of seeing a green light and having to wait a few seconds before you can move?
comment: It's to do with the spaces between the cars. At the lights there's only a small gap, and if all the cars started together then the gap wouldn't grow and if someone hits the breaks there would be an accident. By waiting a second for the car in front to move, you create a safe gap between your car and the car in front.
passages: ACC systems, like human drivers, may not exhibit string stability. This means that oscillations which are introduced into a traffic flow – by braking and accelerating vehicles – may be amplified in the upstream direction. This leads to so-called phantom traffic jams (in the best case) or head-tail collisions (in the worst case). It has been shown that ACC systems designed to maintain a fixed following "distance" will not be string stable. ACC systems designed to maintain a fixed following "time" may or may not be string stable.
   Coordination (the more correct term is progression) refers to the timing of the signals so that a "platoon" of cars traveling on a street arrives at a succession of green lights and proceeds through multiple intersections without stopping. A well coordinated signal system can enhance traffic flow, reduce delay and minimize pollution. However, it is not always possible to retain progression throughout a network of signals. It is also difficult to maintain signal progression on a two-way street. An early traffic engineer Henry Barnes, who served as Commissioner of Traffic in many cities including Baltimore, Maryland and New York City, developed coordinated traffic signal timings, so that large amounts of traffic could be accommodated on major traffic arterials. 
   Prior knowledge and stereotyping plays a role in S–R compatibility. If a required response is inconsistent with a person's stereotyped knowledge of a stimulus and its "typical" reactions, even if the person is aware of the necessary response in the new situation, compatibility will be low. For example, light switches in the United Kingdom are "on" when toggled down, but light switches in the United States are "on" when toggled up; a native of one country visiting the other will demonstrate low S-R compatibility when turning the lights on or off. As another example, red lights are universally associated with "stop" and green with "go", and a reversed configuration will result in a longer reaction time.
   In modern coordinated signal systems, it is possible for drivers to travel long distances without encountering a red light. This coordination is done easily only on one-way streets with fairly constant levels of traffic. Two-way streets are often arranged to correspond with rush hours to speed the heavier volume direction. Congestion can often throw off any coordination, however. On the other hand, some traffic signals are coordinated to prevent drivers from encountering a long string of green lights. This practice discourages high volumes of traffic by inducing delay yet preventing congestion. Speed is self-regulated in coordinated signal systems; drivers traveling too fast will arrive on a red indication and end up stopping, drivers traveling too slowly will not arrive at the next signal in time to utilize the green indication. In synchronized systems, however, drivers will often use excessive speed in order to get through as many lights as possible.
   A mechanism that was developed in Germany is the Grüne Welle, or "green wave", which is an indicator that shows the optimal speed to travel for the synchronized green lights along that corridor. Driving faster or slower than the speed set by the behavior of the lights causes the driver to encounter many red lights. This discourages drivers from speeding or impeding the flow of traffic. See related traffic wave and Pedestrian Crossings, above.
  
  Section::::Organized traffic.:Overtaking.
   While approaching a red signal, drivers may choose to "time a traffic light" by easing off the throttle before the signal. By allowing their vehicle to slow down early and coast, they will give time for the light to turn green before they arrive, preventing energy loss from having to stop.
  
  Due to stop and go traffic, driving during rush hours is fuel inefficient and produces more toxic fumes.
   Traffic signal timing is a very complex topic. For example timing a 'WALK' signal for a wide pedestrian crossing and slower pedestrians (for example the elderly) could result in very long waits for vehicles, and thus increases the likelihood of cars running the light, which could potentially cause accidents. Therefore, optimizing the safety of intersections involves multiple factors like street width, lane width, number of intersecting streets, availability of electricity for a signal, number of cars per unit of time and even/uneven nature of flow, number and type of pedestrians, and many other factors. 
   Another factor that is influencing the ability for a driver-less automobile is the safety of the passenger. To make a driver-less automobile, engineers must program it to handle high-risk situations. These situations could include a head-on collision with pedestrians. The car's main goal should be to make a decision that would avoid hitting the pedestrians and saving the passengers in the car. But there is a possibility the car would need to make a decision that would put someone in danger. In other words, the car would need to decide to save the pedestrians or the passengers. The programming of the car in these situations is crucial to a successful driver-less automobile.
   This can occasionally lead to safety engineering and reliability engineering having contradictory requirements or conflicting choices at a system architecture level. For example, in train signal control systems it is common practice to use a "fail-safe" system design concept. In this example, a wrong-side failure needs an extremely low failure rate as such failures can lead to such severe effects, like frontal collisions of two trains where a signalling failure leads to two oncoming trains on the same track being given GREEN lights. Such systems should be (and thankfully are) designed in a way that the vast majority of failures (e.g. temporary or total loss of signals or open contacts of relays) will generate RED lights for all trains. This is the safe state. This means in the event of a failure, all trains are stopped immediately. This fail-safe logic might, unfortunately, lower the reliability of the system. The reason for this is the higher risk of false tripping, as any failure whether temporary or not may be trigger such a safe – but costly – shut-down state. Different solutions can be applied for similar issues. See the section on fault tolerance below.
   Section::::The problem.
   The scale of this challenge may have other effects. There may be few entities able to marshal the resources and AI capacity necessary to meet it, as well as the capital necessary to take an automated vehicle system to market and sustain it operationally for the life of a vehicle, and the legal and 'government affairs' capacity to deal with the potential for liability for a significant proportion of traffic accidents. This may have the effect of narrowing the number of different system opertors, and eroding the presently quite diverse global vehicle market down to a small number of system suppliers.
   The coordination of the signals is sometimes done dynamically, according to sensor data of currently existing traffic flows - otherwise it is done statically, by the use of timers. Under certain circumstances, "green waves" can be interwoven with each other, but this increases their complexity and reduces usability, so in conventional set-ups only the roads and directions with the heaviest loads get this preferential treatment.
   Traditional gridlock is caused by cars entering an intersection on a green light without enough room on the other side of the intersection "at the time of entering" to go all the way through. This can lead to the car being trapped in the intersection when the light turns green in the other direction. If the same situation occurs simultaneously in multiple intersections, these cars can be trapped in the intersections indefinitely.
   While most trolley conundrums involve hyperbolic and unlikely fact patterns, it is inevitable mundane ethical decisions and risk calculations such as the precise millisecond a car should yield to a yellow light or how closely to drive to a bike lane will need to be programmed into the software of autonomous vehicles. Algorithms dictate, for example, how closely to drive to a bike lane or the precise moment an autonomous car should yield to a yellow light. Mundane ethical situations may even be more relevant than rare fatal circumstances because of the specificity implicated and their large scope. Mundane situations involving drivers and pedestrians are so prevalent that, in the aggregate, produce large amounts of injuries and deaths. Hence, even incremental permutations of moral algorithms can have a notable effect when considered in their entirety.
   Unlike typical developed nations that have only a handful of vehicle types on their roads, many types of transport will share the same roads as cars and trucks; this diversity is extreme in Delhi, India, where more than 40 modes of transportation regularly use the roads. In contrast, New York City, for example, has perhaps five modes, and in parts of America a vast majority of traffic is made up of two types of vehicles on the road, cars and trucks.
   Any vehicle travelling along with the green wave (at an approximate speed decided upon by the traffic engineers) will see a progressive cascade of green lights, and not have to stop at intersections. This allows higher traffic loads, and reduces noise and energy use (because less acceleration and braking is needed). In practical use, only a group of cars (known as a "platoon", the size of which is defined by the signal times) can use the "green wave" before the time band is interrupted to give way to other traffic flows.
   In 2017, Researchers from Arizona State University developed a 1/10 scale intersection and proposed an intersection management technique called Crossroads. It was shown that Crossroads is very resilient to network delay of both V2I communication and Worst-case Execution time of the intersection manager. In 2018, a robust approach was introduced which is resilient to both model mismatch and external disturbances such as wind and bumps.
  
  Among connected cars, an unconnected one is the weakest link and will be increasingly banned from busy high-speed roads, predicted a Helsinki think tank in January 2016.
  
  Section::::Public opinion surveys.
   Meanwhile, in the United Kingdom, lights that change to red when sensing that an approaching motorist is traveling too fast are being trialled in Swindon to see if they are more effective at reducing the number of accidents on the road than the speed cameras that preceded them and which were removed following a council decision in 2008. These lights are more focused on encouraging motorists to obey the law but if they prove to be a success then they could pave the way for more sophisticated systems to be introduced in the UK.
  
  Section::::Previous research.
   Section::::Types and placement of colours.:Traffic signal cycles.
  
  Generally, at least one direction of traffic at an intersection has the green lights (green aspect) at any moment in the cycle. In some jurisdictions, for a brief time, all signals at an intersection show red at the same time, to clear any traffic in the intersection. The delay can depend on traffic, road conditions, the physical layout of the intersection, and legal requirements. Thus modern signals are built to allow the "all red" in an intersection, even if the feature is not used.
   With technologies in developed countries continuing to advance, there is now an increasing move to develop and implement smart traffic lights on the roads. These are systems that adapt to information that is received from a central computer about the position, speed and direction of vehicles. They try to communicate with cars to alert drivers of impending light changes and reduce motorists' waiting time considerably. Trials are currently being conducted for the implementation of these advanced traffic lights but there are still many hurdles to widespread use that need to be addressed; one of which is the fact that few cars yet have the required systems to communicate with these lights.
   Pedestrians are usually not able to get through the intersection with one green light. It can take up to four cycles to walk through the entire length of a SPUI.
   MIN time determines the minimum duration of the green interval for each movement. Left turns, minor streets, major streets, usually have different MIN times. Left turns and minor side street intervals are often in the range from 4 to 10 seconds while major streets often go higher than 15. 
   In New Zealand, paired red/red traffic lights are often installed outside fire and ambulance stations on major roads, which when activated by the station, flash alternatively (so that at any time one red light is always showing), the purpose being to cause traffic to stop for a set amount of time to allow emergency vehicles to exit their station safely.
   BULLET::::2. Redundant copies of some signals that are of particular safety importance such as displacement and force measurements of the brake pedal copied by multiple processors in the pedal interface unit.
  
  BULLET::::3. Redundant hardware to perform important processing tasks such as multiple processors for the ECU in Fig. 1.
  
  In order to utilize the existing redundancy, voting algorithms need to be evaluated, modified and adopted to meet the stringent requirements of a brake-by-wire system. Reliability, fault tolerance and accuracy are the main targeted outcomes of the voting techniques that should be developed especially for redundancy resolution inside a brake-by-wire system.
   Mechanical vehicle switching minimizes inter-vehicle spacing or headway distance, but it also increases the minimum distances between consecutive junctions. A mechanically switching vehicle, maneuvering between two adjacent junctions with different switch settings, cannot proceed from one junction to the next. The vehicle must adopt a new switch position, and then wait for the in-vehicle switch's locking mechanism to be verified. If the vehicle switching is faulty, that vehicle must be able to stop before reaching the next switch, and all vehicles approaching the failed vehicle would be affected.
  