Fixed, reactive and predictive systems

There are two different metering approaches available. First is Pre-timed metering, which use fixed signal cycles. Second is Traffic responsive, which uses real time traffic data to calculate signal cycle lengths. Traffic responsive systems can be local or system-wide.

Pre-timed (fixed) systems

In the pre-timed ramp metering systems, the ramp signal operates with a constant cycle in accordance with a metering rate prescribed for the particular control period.. the salient features of this type of ramp metering are:

It is the simplest and least expensive form of ramp metering for construction and installation.
It is also the most rigid approach because it cannot make adjustments for real-time conditions including non-recurring congestion (i.e., congestion that occurs as a result of weather, collisions, etc.).
Th system being pre-timed, it is best used to address conditions that are predictable from day-today.
If there is no mainline or ramp detection, agencies must regularly collect data by alternative means in order to analyze traffic conditions on the freeway and determine the appropriate metering rates.
The metering operation will require frequent observation so that rates can be adjusted to meet traffic conditions which is a drawback.

Traffic responsive systems

In contrast to the pre-timed metering control, traffic-responsive metering is directly influenced by the mainline and ramp traffic conditions during the metering period. Metering rates are selected on the basis of real-time measurements of traffic variables indicating the current relation between upstream and downstream capacity. The salient features of this type of ramp metering system are:

This system uses freeway loop detectors or other surveillance systems to calculate or select ramp metering rates based on current freeway conditions.
It is generally considered to be five to ten percent better than those of pre-timed metering.
A traffic responsive approach can be used either locally or system-wide.

Local traffic responsive

Local ramp metering is employed when only the conditions local to the ramp (as compared with other ramps) are used to provide the metering rates. The salient features are:

Local traffic responsive metering approaches base metering rates on freeway conditions near the metered ramp.
This is used where the traffic congestion at a location can be reduced by the metering of a single ramp.
They are used as backups when system-wide algorithms fail.
Unlike pre-timed systems, local systems require surveillance of the freeway using traffic detectors.
Although, more capital costs are required to implement traffic responsive systems, they more easily adapt to changing conditions and can provide better results than their pre-timed counterparts.

System-wide traffic responsive

In most cases, it is preferable to meter a series of ramps in a freeway section in a coordinated fashion based on criteria that consider the entire freeway section. The strategy may also consider the freeway corridor consisting of the freeway section as well as the surface streets that will be affected by metered traffic. The salient features are:

This is used when there are multiple bottlenecks or locations of recurring congestion along a freeway.
This type of ramp metering is used to optimize traffic flow along a metered stretch of roadway, rather than at a specific point on the freeway (as is the case of local traffic responsive systems).
Like local traffic responsive systems, system-wide traffic responsive systems require data from ramp detectors and local freeway detectors.
In addition to these components, system-wide traffic responsive systems are unique in the fact that data is also needed from downstream detectors and/or upstream detectors at multiple locations, potentially from cross-street signal controllers, and from the central computer.
System-wide traffic responsive systems have the most complex hardware configuration compared to the other metering approaches discussed so far (i.e., pre-timed and local traffic responsive).