Who makes the decision to put a stop light at an intersection? What would they/he/she use to determine the need?
Around here it’s mainly the city’s transportation department that decides with help from the state. They do traffic studies measuring amount of traffic on main and cross streets, they look at accident rates for areas, they get ‘encouraged’ by lawsuits, and they review reports from the insurance industry. However, these are the same people that took a major 4 lane East/West through street and cut it down at the busiest stretch to 1 lane in each direction to make a left turn lane for the 1 in 80+ people that turn there. If it is a state highway then it is joint control (the highway is technically the state’s responsibility, but the cross streets are the local government’s. Private companies can request permission to install them as well. 3M installed and maintains their own set of stoplights along the major streets that pass their main corporate campus (and they sometimes experiment with new stoplight technologies there as well). If a neighborhood decides that an area is becoming dangerous (traffic running through too fast, too much traffic on side streets, etc.) they can petition the City Council to install a light or stop sign to slow things down.