Investigation of Freeway Capacity: a) Effect of Auxiliary Lanes on Freeway Segment Volume Throughput and b) Freeway Segment Capacity Estimation for Florida Freeways

Auxiliary lanes are generally used to reduce the traffic turbulence created by merging and diverging movements and are primarily used by vehicles either entering or exiting the freeway. The Highway Capacity Manual (HCM) does not offer explicit guidance on the benefit of adding an auxiliary lane between an on- and off-ramp. The objective of this part of the project was to quantify the additional traffic volume that can be accommodated on a freeway segment by connecting an on-ramp to an off-ramp with an auxiliary lane. The approach used was to identify the traffic volume level at which each level of service density threshold was met for the conditions of with and without an auxiliary lane. The CORSIM simulation program was used to generate the data upon which to establish the quantitative effect of an auxiliary lane. Two versions of an adjustment equation that gives the percentage increase in volume throughput due to adding an auxiliary lane were developed. The developed equations are simply a function of the number of mainline lanes, as other factors were not found to significantly affect the percentage increase in volume throughput. The capacity of a freeway segment is a critical factor for the assessment of the traffic flow operations on freeway facilities. The HCM (2000) is considered to be one of the authoritative sources on capacity values for a variety of roadway types in the U.S. It provides a single set of capacity values for basic freeway segments as a function of free-flow speed. These values are considered to be reasonably representative values for freeways located throughout the U.S., but it is recognized that lower or higher values may be more appropriate in any given location. However, the HCM does not provide any guidance on how its recommended values can be adjusted to reflect significant differences in capacity due to local conditions, nor how to directly measure or estimate capacity values. The objective of this part of the project was to investigate various methods that can be used to arrive at an estimate of freeway capacity values, and to recommend one of these methods to the Florida Department of Transportation for use in developing its own estimates of capacity for Florida freeways. Three methods were investigated: one that fits a mathematical function to speed-flow data points, from which the apex of the function is taken as capacity; one that estimates a breakdown probability distribution based on flow rates preceding breakdown events, from which capacity can be taken to correspond to a certain percentile value of the breakdown probability distribution; and one that uses a flow rate corresponding to a specified percentile within a specified range of maximum flow rates observed at a site. It is recommended that this latter method is most suitable for planning and preliminary engineering applications.


  • English

Media Info

  • Media Type: CD-ROM
  • Edition: Final Report
  • Features: Appendices; Figures; Photos; References; Tables;
  • Pagination: 162p

Subject/Index Terms

Filing Info

  • Accession Number: 01154796
  • Record Type: Publication
  • Report/Paper Numbers: TRC-FDOT-73157-2010
  • Contract Numbers: BDK-75-977-08
  • Created Date: Apr 15 2010 1:09PM