## 2 Traffic Management and Ramp Metering  
In the last few years, traffic management techniques have been improved, tested and deployed. Based on those successes, the Highway Capacity Manual [7] defined the Active Traffic Management (ATM) as \"a comprehensive approach optimizing the operational performance of the roadway system through monitoring and control of systems operations.\" This approach can be described with RM, dynamic lane use or shoulder control, dynamic speed limits, queue warning, dynamic rerouting, dynamic junction control, and adaptive traffic signal control. What's more, these can influence both the nature of the facility's demand and ability to deliver the capacity to serve the demand. Therefore, ATM measures can improve facility performance, sometimes significantly. ATM practices are not widespread in Saudi Arabia. However, some traffic management technologies have been recently featured in a few freeways and arterials in Riyadh. These are dynamic speed limits, queue warnings and dynamic reroutings on King Abdullah Road, Abu Bakr Al-Siddiq Road, and Al-Urubah Road. There is also a SAHER system used to enforce speed limits and red lights at signalized intersections. Among the various applications of ATM, RM was proven to maximize the capacity of freeway systems by a safe and efficient movement of traffic [8]. Simply put, RM is the use of a traffic signal installed on an on-ramp to control the entrance rate of vehicles onto the freeway. Since multiple vehicles attempt to merge onto the mainline at once, drivers on the freeway mainline tend to reduce their speed to allow vehicles to enter the freeway and a succession of these involuntary yields may cause lines. However, if vehicles enter the mainline at controlled intervals, they are less likely to disrupt the traffic flow on the mainline and more likely to increase average speed and shorten overall travel time on the freeway. RM also reduces the number of collisions, which are often caused by several vehicles merging from the on-ramp [8]. There are two types of controls in RM. The first is fixed-time control, where meter rates are based on historical conditions and are fixed according to the time of the day. Ramp meters work based on pre-set schedules. The pre-set metering is a fixed metering rate that is not influenced by current mainline traffic conditions. This rate is usually set based on historical data. This RM is generally operated with a two--(red and green) or a three--(red, yellow and green) signal displayed by a local controller that can change metering rates by the time of the day and accept detector inputs. Although the ramp signal operates with a constant cycle length, the timing of the green, yellow, and red intervals varies on the type of metering: single-entry metering or platoon metering. Compared with single-entry metering, platoon metering is more complex in operation and may cause some driver confusion, which may lead to disruptions of ramp flow. What's more, platoon metering has been successfully implemented in several locations. The second control type of RM is a traffic-responsive control, which requires real-time mainline traffic data to determine the control parameters. This data collection needs more detectors than the fixed time RM. Based on real-time traffic data measurements, the maximum RM rate at which vehicles can be permitted to enter the freeway is determined. Lane occupancy (a surrogate measure for density) and flow rate (volume) are two important traffic variables (control parameters) in deciding an operational strategy in the traffic-responsive RM. Various algorithms are available for the traffic-responsive RM, such as demand-capacity algorithm, percent-occupancy algorithm, ALINEA algorithm, speed control algorithm, and gap occupancy algorithm. Three widely referenced local metering algorithms are demand-capacity, percent-occupancy, and ALINEA algorithms. This research uses the ALINEA algorithm since it is the most popular algorithm for the traffic-responsive RM [9; 10; 11; 12; 13].