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Capacity and Level of Service at Unsignalized Intersections Final Report Volume 1 - Two-Way-Stop-Controlled Intersections (1996) / Chapter Skim
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Chapter Five. Critical Gap and Follow-up Time
Pages 55-64

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From page 55...
... These attributes included geographic sector, number of legs, multilane or single lane on major street, number of lanes a minor movement crosses, major street volumes, major street right turn percentage, percentage of traffic on the conflicting lane for multi-lane sites, minor movement with shared lane or exclusive lane condition, Once oftwo-way left-turn lane on the major street, distance to upstream signals, grade of minor approach, and average vehicle delay. The database used for the regression analysis was provided as Appendix m of Working Paper 16 (NCHRP 3-46, 1995~.
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From page 56...
... This is perhaps a better indicator of the difficulty of a movement maneuver compared to using just a multi-lane or single lane. For example, be existence of exclusive left turn lanes on the major street requires Me minor streetleR turn movement to cross atleast a two-lane width and the minor street through movement to cross at least a four-lane width, which may prove to be more difficult .
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From page 57...
... Through We regression analysis, Me following conclusions were reached: . The following factors that have significant effects on die critical gap were identified: Anal - whether the intersection has multilane or single lane approaches on the major street SORT - percentage right turn movement on the major street NoLeg - whether Me intersection has 3 legs or 4 legs Grade - the grade of the minor steam approach Tu~nAngle - whether the minor stream movement has a small turn angle The regression analysis included some sites Ninth unusual geometric charactenstics, such as a one-way major street or a twoway left-turn lane.
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From page 58...
... Alternatively, 36.ss the cntical gaps could be related to the major street flow so that the iteration could be avoided. However, since the cntical gap is also related to over factors, the relationship between major street flow rate and cntical gap is always difficult to establish.
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From page 59...
... Table 25 lists the results measured for heavy vehicles, for passenger cars, and for all vehicles grouped together. The measurements are grouped based on intersection geometry and vehicle movement type.
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From page 60...
... Weighted Average Values of Follow-Up Times Measured for Passenger Cars and Trucks The Effect of Direction of the Major Street Flow In the max~m~ ~e~ihood procedure, ~e maximum rejected gap of each vehicle dete~m~nes the lower limit of the critical gap value. To investigate ~e effect of different major street movements on the minor street driver, the maximum rejected gaps were extracted and classified by different ending gap movement type.
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From page 61...
... Table 27 lists He ratio of the follow-up time and the cntical gap measurements for each movement. Figure 29 shows the relationship between cntical gaps and the follow-up times measured for all He sites.
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From page 62...
... . major street volume or minor stream vehicle delay intersection geometry, including the number of lanes on Be major street and the number of legs of the intersection right turn volume proportion on Be major street minor stream approach grade movement turn angle With the increase of major stream volume or minor stream vehicle delay, He critical gap and follow-up time tend to decrease.
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From page 63...
... Combined critical gap is computed based on the proportion of passenger cars and heavy vehicles Table 29. Recommended Follow-Up Time Values ,..
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Key Terms

  • major street
  • minor street
  • heavy vehicles
  • street left
  • movement type

  • cntical gap
  • passenger cars
  • intersection geometry
  • critical gaps
  • vehicle delay

  • gap tends
  • minor stream
  • gap value
  • regression analysis
  • minor movement

  • street driver
  • gap measured
  • rejected gaps
  • maximum rejected
  • delay effect

  • street flow
  • street vehicles
  • followup time
  • conflicting lane
  • approach grade

  • site usually
  • vehicle type
  • traffic flow
  • time measurements
  • minor sheet


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