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Negative Densities on Freeway Merges and Diverges

Andrew Bell
URS - North Carolina
Negative Densities on Freeway Merges and Diverges was posted on July 14th, 2014 at 9:07 AM EDT
regarding Chapter 13: Freeway Merge and Diverge Segments

Dear TRB,

During a recent capacity analysis involving Freeway Merge and Diverge Segments, we obtained results which included negative densities. We typically obtained these results for ramp areas with lower volumes and longer acceleration and deceleration lane distances. Upon review of the formulas used to calculate the density for On-Ramp Influence Areas (Eq. 13-21) and Off-Ramp Influence Areas (Eq. 13-22), it was confirmed that it is indeed possible for a Freeway Merge or Diverge analysis to result with a negative density.

While a negative density simply results in a level of service of A, it is difficult to present a negative density as a result simply because it cannot and does not occur in the field. It would be beneficial to our practice for an amended formula to be written that places a lower density limit for Freeway Merges and Diverges, or for a statement to be added that states that the minimum density should be zero.

Thanks for your time,

Andrew Bell

Scott Washburn
University of Florida
RE: Negative Densities on Freeway Merges and Diverges was posted on September 11th, 2014 at 9:45 PM EDT

Dear Andrew:

Please send your analysis details to me ( and I will make sure that the freeways subcommittee examines this problem. Also let me know if your analysis was done using a particular software package or manual calculations, and if you had reviewed all of the errata material for chapter 13.

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