In the simultaneous system, all signal lights are changed at a fixed time, and are the same on each street in a given direc tion; i.e., green or clear lights are shown permitting all north and south traffic to move throughout the controlled area, while the red or "stop" signal is set at crossings for east and west movements. Then, after a fixed interval, the signals and the corresponding traffic movements are reversed. This system is the simplest form and is used with some modifications in many cities.
The progressive system of operation is designed to keep all traffic moving continuously in a given direction. It is usually applied to the heavier thoroughfares where the cross-traffic is relatively light.
By this system signals are changed on one street so that a green light is seen at the beginning of one traffic block, and as the car progresses at a predetermined speed the red light at the other end of the block changes to green, permitting a continuous move ment at from 20 to 3omi. an hour.
Many variations of the simultaneous and progressive systems have been worked out to fit special districts, and are operated by some form of electric control at a central point from which cables are run through to the various lights so that the timing of the intervals can be changed as desired by adjustment at the central station. The complete cycle used generally varies from 3o to 5o seconds.
The use of automatic control signals has been extended to cover intersections on main highways such as State highways, county roads, and other main arteries, in order to protect vehicles from collision where high speeds are permissible on the longer reaches of main thoroughfares. The very high speeds possible with modern motor vehicles have necessitated the introduction of vari ous devices to separate lanes of traffic and to avoid intersec tions at grade. One of the most common of these expedients is the division of major highways by a central dividing strip to separate opposing lines of traffic. At intersections, the "clover leaf" form of route separation has been developed in a number of different ways—the process being to confine cross traffic to roadways passing under or over the intersection. Cars enter or leave one or other of the main highways by merging or diverg ing right turns, thus avoiding crossing opposing traffic at grade.
Another form for regulating traffic at intersections is by means of a traffic circle in which all traffic must pass to the right and avoid direct intersection of opposing traffic by rotary movement around the circle and leaving by a right turn at any desired point.
Another regulation at intersections, which has become generally used, is the "Full Stop" sign requiring all vehicles entering from a minor thoroughfare to come to a full stop before entering the main thoroughfare.
One-way traffic on selected streets in cities, and regulation of right and left turns at street intersections, have been resorted to in a variety of ways by different traffic officials. A continuous effort has been made to establish standard methods throughout the country so that the type and operation of signals will be recognized correctly by drivers and the proper operation of their cars accomplished with the least possible delay.
Maximum speeds on public highways have been steadily in creased until in some States somi. an hour is legal, and in other States the speed is unlimited between cities but the responsibility for accidents is placed severely upon the car operator. Local speed limits are generally displayed on roadside signs by munici palities and other public authorities.
One of the financial problems met in large cities in traffic con trol is the cost of keeping police officers on post at intersections. Automatic traffic lights in many cases effect a great reduction in the police force required. However, on heavy thoroughfares in our larger cities, many officers are still required to see that drivers promptly and correctly obey the traffic lights and to pro tect and regulate movement of pedestrians. In some cities pedes trian regulation is as rigidly enforced as vehicle movements at the principal crossings. To meet special conditions at certain hours of the day an officer is sometimes posted at an intersection to control signals by hand that are automatically operated during the remainder of the 24 hours.
Traffic regulation is also affected by the rules as to parking of vehicles on public streets. There are many views taken of this privilege, relative to the interest of automobile drivers, stores, and other buildings desiring access from automobile patrons. On the other hand the narrowing of the streets by solid lines of parked vehicles seriously interferes with vehicular movements. In some cities absolute prohibition of parking in business districts has been enforced.
In some cities meters have been installed for payment for park ing at the street curb. (H. M. BR.)