Arrival Turning Flows
Average Daily Traffic
Circulating Width C
Circulating Lanes nc
The diameter D is the outer diameter or Inscribed Circle Diameter.
HCM does not use the D.
The relationship between capacity and D is more complicated than other parameters.
For a given circulating flow the increase in capacity, shown below, is moderate.
However, increasing D gives greater increases in capacity the larger the circulating flow as shown below.
Capacity is a small when opposing flows are large, so having a geometric parameter that increases capacity most when capacity is least is very useful.
However, moderately increasing D increases the ROW disproportionately. On the contrary, moderately reducing the D can release enough land to widen entries and increase flare lengths sufficiently to achieve a large net gain in capacity.
The effect of varying D should be checked with the accident models.
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