Analysis of 2+1 Roadway Design Alternatives

One of the main problems encountered when analyzing 2+1 design is to determine the optimum length and frequency of additional lanes. NCHRP recommends that passing lane lengths on 2+1 roadways should be consistent with optimal lengths for isolated passing lanes on two-lane highways. However, it is not reported that any special analysis was carried out in this aspect. Then, it was decided to analyze different layouts of 2+1 design using Twopas simulation model. The model was previously calibrated for traffic conditions on Polish roads. The analysis was done for an 8.4 km long section with fixed-length additional lanes located continuously in alternating directions. The analyzed alternative module lengths were: 600, 950, 1300 and 2000 m. Two measures of effectiveness were analyzed: percent time spent following (PTSF) and total vehicle-hours. Two-lane two-way section with passing allowed throughout the whole length was assumed as the base-case. The parameters of the analysis were: traffic volume and percent of trucks. As a result of the analysis the relationships between traffic volume and PTSF and time savings were derived. The relationships show that for any module length from 950 to 2000 m there is no significant difference in the measures of effectiveness for the whole range of traffic volumes analyzed (from 250 to 1500 veh/h/dir), whereas for the 600 m module the calculated values are slightly worse. Thus, in practice any module length from the range of 1000-2000 m can be used, regardless of traffic volume. These results are different from the ones presented by NCHRP.


  • English

Media Info

  • Media Type: CD-ROM
  • Features: Figures; References; Tables;
  • Pagination: 15p
  • Monograph Title: 3rd International Symposium on Highway Geometric Design, June 29-July 1, 2005, Chicago, Illinois: Compendium of Papers CD-ROM

Subject/Index Terms

Filing Info

  • Accession Number: 01004394
  • Record Type: Publication
  • Files: TRIS, TRB, ATRI
  • Created Date: Sep 21 2005 2:40PM