Chapter 13 - Traffic Forecasting and Analysis Concepts

13.2. Freeway Traffic Analysis and Design

Traffic Analysis and Design

The purpose of this section is to provide some traffic analysis guidance for design engineers on some of the factors and design elements to consider in operational and road capacity analysis. This information is intended as a supplement to GDOT adopted standards and procedures outlined in the Transportation Research Board (TRB) Highway Capacity Manual (HCM).

The Transportation Research Board (TRB) Highway Capacity Manual provides comprehensive guidelines related to freeway traffic analysis and design. Some considerations that must be made during the traffic analysis and design process include, but are not limited to:

• A freeway experiencing extreme traffic congestion differs greatly from a non-freeway facility experiencing extreme congestion since the travel conditions creating the congestion are internal to the facility, not external to the facility.
• Freeway facilities may have interactions with other freeway facilities in the area as well as other classes of nearby roads, and the performance of the freeway may be affected when travel demand exceeds road capacity on these nearby road systems. For example, if the street system can not accommodate the demand exiting the freeway, the over-saturation of the street system may result in queues backing onto the freeway, which adversely affects freeway travel.
• The traffic analysis and design process must also recognize that the freeway system has several interacting components, including ramps and weaving sections. The performance of each component must be evaluated separately and their interactions considered to achieve an effective overall design. For example, the presence of ramp metering affects freeway demand and must be taken into consideration in analyzing a freeway facility. 
• High occupancy vehicle (HOV) lanes require special analysis.  If an HOV facility has two or more lanes in each direction all or part of the day and if access to the HOV facility is limited from adjacent freeway lanes (i.e. 1 mile or greater access point spacing), these procedures may be used. Otherwise, HOV lane(s) will have lower lane capacities.