50%-60% of the delay on urban freeways is associated with Incidents. Debris on the road, flooding, cargo spills, vehicle collisions or accidents, special events such as ball games and festivals, and highway work are examples of incidents. Timeliness is the key to incident clearing and the minimization of congested flow conditions. Rapid detection and verification allow faster and perhaps less complex response options and more effective aid to victims.

The most common technique for addressing recurring congestion on freeways is ramp metering. It limits the rate at which vehicles enter the freeway's mainline so that the downstream mainline capacity is not exceeded.

• The sensors on the mainline serve a dual purpose: adjustment of the ramp-metering rate in response to real-time demand and collection of historical volume and occupancy data.
• A demand sensor on the ramp indicates the arrival of a vehicle at the stop line and the commensurate start of the metering cycle. Demand at the stop line is typically required before the ramp signal is allowed to turn green.
• A passage sensor detects when the vehicle passes the stop line and returns the ramp signal to red for the next vehicle. The passage sensor can also be used to monitor meter violations and provide historical data about the violation rate at each ramp.

In this strategy, traffic flow data are analyzed at a central traffic management center that simultaneously adjusts the metering rates at several ramps. The metering rates are found from the analysis of the demand and capacity of an entire freeway section rather than traffic conditions in the immediate vicinity of individual ramps.

Mainline metering is one form of freeway mainline traffic management that also makes use of driver information systems, variable-speed control, lane closure, and reversible lane control. Individually or in combination, these control techniques are gaining acceptance at many highly congested freeway locations throughout the world.

The desired level of service for mainline metering is selected to achieve one or several of the following objectives:

• Flow maximization through a downstream bottleneck.
• High level of service downstream of the control point.
• Distribution of total delay on the freeway system more equitably.
• Diversion of traffic to other routes or modes.
• Increase in the overall safety of the facility.

Another technique for managing recurring congestion is to meter freeway-to-freeway connector ramps. Experiences indicate that significant benefits can be achieved with connector metering under conditions similar to those associated with ramp metering.

Safety problems occur when the design speed for certain curves is below that of other portions of the freeway. To lessen the crash potential at such locations, a speed measurement system that incorporates a flashing display to alert the driver to an unsafe speed can be used. Thus, if a vehicle is traveling faster than the desired or safe speed, a flashing sign or signal is activated to advise the driver to reduce the speed of the vehicle.

Reduce congestion inside the tunnel, ensure that the vehicles in the maximum queue can exit the tunnel exit within one cycle, and maximize the elimination of safety hazards inside the tunnel caused by accidents and other incidents.