Traffic Signals:
The overall objective of signal control is to provide for safe and efficient traffic flow through intersections, along routes, and in street networks.
A traffic signal can provide one or more of the following benefits:
Improve the orderly movement of traffic
Interrupt heavy traffic at intervals to allow pedestrian and side street traffic to cross or enter the main street traffic stream
Increase the traffic-handling capability of the intersection
Reduce the frequency of certain types of accidents

Whether or not a traffic signal is needed, its installation will probably have the following negative impacts:
Increased delay to major traffic movements
Increased frequency of certain types of accidents (principally rear-end accidents)
Reduce freedom for motorists to control their own progress

If an intersection really needs a signal, these disadvantages will be offset by the benefits just listed. If however, a traffic signal is not really needed, its installation can have the following unnecessary additional adverse effects:
Excessive delay to all traffic movements including those the signal may have been installed to help.
Unnecessary frustration for motorists, possibly resulting in disobedience of the traffic signal.
Undesirable diversion of traffic through neighborhoods

Traffic Signals Brochure

Signing:
Traffic control devices such as signs and markings are the primary means of regulating, warning or guiding traffic on all streets and highways. The need for well designed, adequately maintained devices grows in proportion to the density of traffic, speed of operation, and complexity of maneuvering areas on highways and at intersections.

Both signs and markings have the function of regulating, warning, guiding and/or channelizing traffic. To be effective, however, the installation of each device should fulfill a need, command attention, convey a clear simple meaning, command respect of road users, and give adequate time for proper response.

The traffic engineer must employ five basic consideration to ensure that these requirements are met:

Design: the combination of physical features such as size, colors, and shape to command attention and convey a message.

Placement: the installation of devices so that they are within the cone of vision of the user and thus command attention and give time for response.

Operation: the application of devices so that they meet the traffic requirements in a uniform and consistent manner, fulfill a need, command respect and give time for response.

Maintenance: the upkeep of devices in order to retain legibility and visibility; the removal of devices if not needed in order to aid in commanding respect and attention while fulfilling the needs of the users.

Uniformity: the uniform application of similar devices of similar situation so that they fulfill the need and command his respect.