MDC ATMS Frequently Asked Questions
What is the purpose of a traffic signal?
Traffic signals are electronically operated traffic control devices which alternately direct traffic to stop and to proceed.
Traffic signals are designed to ensure an orderly flow of traffic, provide an opportunity for pedestrians or vehicles to cross
an intersection and help to reduce the number of conflicts between vehicles entering intersections from different directions.
How do traffic signals work?
Traffic responsive controllers change the signal displays according to the amount of traffic in each direction. These controllers use
sensors (inductive loops in the roadway or overhead cameras) to detect the number of vehicles and automatically adjust the length of the green time
to allow as many vehicles as possible through the intersection before responding to the presence of vehicles on another approach.
Although these types of traffic controllers have been in use for many years, a new generation of microcomputer traffic controllers
makes the signalized intersection much more efficient, thereby reducing time-consuming delays.
Why do some traffic signals take so long to change (i.e. Why do I have to wait so long after I stop at a specific signal)?
Signal timing cycle lengths usually fall between 40 and 180 seconds. While longer cycle lengths do yield longer red displays, they have the advantage of handling higher volumes of traffic and improving coordination with adjacent signals. The timing for each signal is determined based on traffic volume
and traffic patterns in each particular area. Sometimes traffic flow patterns change due to community growth or changes in nearby land use. At most traffic signals several different timing plans are used throughout the day to account for varying levels of traffic demand.
The length of the wait depends on the signal cycle length and amount of traffic. In general, a longer cycle length increases the amount of vehicles that
can be moved through an intersection (capacity). Increasing cycle lengths also increases driver delay. Cycle lengths must be longer at larger
intersections to serve the greater number of separate traffic movements during the timing sequence, to accommodate much longer pedestrian crossing
times, and to accommodate higher volumes of traffic. Requests for timing changes at individual intersections should be referred to the Traffic Department.
Information needed for a signal employee to investigate a requested timing change is what day of the week and what time of day a problem occurs.
Why are signals not always perfectly synchronized?
Intersection signals are coordinated, or synchronized with each other to reduce stops and delay for the major traffic movements.
Coordinating signals require that all signals be programmed with a common cycle length, which is the amount of time it takes a signal to
sequence through all traffic movements one time. The quality of movement through a series of traffic signals depends on the spacing
between signals, the speed of traffic, the cycle length, and the amount of traffic. Signals along main arterials are generally coordinated
with each other during the day, when there are heavy traffic flows. It is often not possible to progress traffic in both directions because
of imperfect spacing between traffic signals. Sometimes it is necessary to choose one direction to progress. When two-way progression is not possible,
the Traffic Operation Engineer often uses computerized traffic modeling to find coordinated timing plans that decrease the total delay and stops for all users
of the system. Traffic turning onto of off of a side street is generally not progressed, and turning vehicles can usually expect to stop at the
Why isn't there enough green time to get the traffic through the signal for a specific movement?
The amount of green time programmed for each movement at a signal varies by time of day. Sometimes there is more traffic at a signal than the signal can handle,
and the signal is over its capacity. In these situations, the Traffic Operation Engineer attempts to time the signals to equalize delays for conflicting movements.
At other times green time can be moved from one movement to a conflicting movement, realizing that improving one movement hurts another. Increasing green for
one movement requires decreasing the amount of green time for another movement.
Requests for timing changes at individual intersections should be referred to the Traffic Department.
The information needed for a signal operator to investigate a requested change is what day of the week and at what time of day a problem occurs.
How does Miami-Dade County Public Works and Waste Management decide whether a traffic signal should be installed on a State Highway?
Miami-Dade County Public Works and Waste Management follows federal guidelines (Manual on Uniform Traffic Control Devices - MUTCD) that establish minimum conditions under which a signal
installation should be considered.
The MUTCD contains criteria (warrants) which are used to define
the need for, and appropriateness of a particular traffic control device. These warrants are usually expressed in the form of numerical requirements such
as the volume of vehicular or pedestrian traffic.
Warrants should be viewed as guidelines, not as absolute values. However, if no warrants are met, a signal will not be installed. Satisfaction of a warrant
is not a guarantee that the signal is needed. The warrant analysis process is just one of the tools to be used in determining if a traffic signal is necessary.
Engineering judgment should be exercised in making the final determination.
How do I get a new signal installed?
Installing a new traffic signal first requires determining if a signal is needed. If a signal is needed, then a method of funding and constructing
the signal must be found. Evaluating the need for a traffic signal requires careful analysis of the accident history, the intersection geometry,
and amount of traffic. The number of traffic accidents almost always increases when a signal is installed, as interruption of free flow results in
an increased number of rear-end type accidents. Certain types of accidents that tend to be more severe can often be reduced by installing a signal.
The analyst must weigh the different types of accidents and compare them to federal guidelines. There are certain federal requirements that must be
met before a signal can be installed.
Why does it take so long to get a traffic signal installed once it is approved?
If the signal is to be installed by a Contractor as a signal project, then it is added to a list of countywide locations which are competing for funds.
These funds must be allocated well in advance which results in projects often "waiting" on the list for several years continually competing for
funds against other projects which may have a greater need.
Signals can also be installed by local governments in a joint effort with the Department of Transportation.
Will traffic signals reduce intersection crashes?
Traffic signals do not prevent crashes. Certain types of crashes can be reduced in number or severity by the installation of a signal, while other types
will increase. Where signals are used unnecessarily, the most common results are an increase in total crashes, especially rear-end collisions.
Traffic signals are not an answer for every problem intersection. A signal placed at a wrong location can contribute not only to rear-end collisions, but
excessive delays, unnecessary travel on alternate routes and more congested traffic flow.
Why are there not Stop Signs at more intersections?
Stop signs are one of the most effective sign control devices when used at the right location and under the right conditions. The Manual on Uniform Traffic
Control Devices defines the appropriate use of stop signs and stresses that they are not to be used for speed control. A stop sign is intended to help assign
right-of-way for drivers and pedestrians at an intersection.
Stop signs that arbitrarily interrupt traffic or cause such an inconvenience as to force traffic to use other routes are “nuisances” and typically
cause a high incidence of intentional violation. When vehicles do stop at warranted stop signs, the speed reduction is effective only in the immediate vicinity
of the stop sign, and frequently speeds are higher between intersections.
Why don't we just reduce the speed limit?
Setting the speed limit below what the majority of motorists are driving does not reduce the speed of traffic. Most drivers choose a speed that feels right given
the condition of the road. If we set the limit lower than what the driver feels it should be, it will be ignored by most.
How do I report a malfunctioning traffic signal?
contact the Miami-Dade County Traffic Control Center at firstname.lastname@example.org or 305-592-8925. A traffic operator will investigate and notify the proper individual(s) to make any necessary repairs or adjustments.
When you call, please give us specific information about the location of the signal so we can make the repair as quickly as possible.