Back Continue

HCMAG Home
Overview
Introduction
Getting Started
Problem 1
Problem 2
Problem 3
Problem 4
Problem 5
Problem 6
Problem Index
Datasets
Search

 

Sub-problem 4b - Page 3 of 5

ID# C204B03

Sub-problem 4b: Clifton Country Road PM peak hour - Existing Conditions

Demand vs. Volume
When you collect turning movement counts at an intersection, you are recording the output from the intersection, but not necessarily the input. Traffic counts record the vehicles that have been processed by the intersection, leaving uncounted those that are queued up and waiting to be processed. The counts you collect are volumes, and the arrivals are demand. In many cases, congested intersections have demand/volume ratios greater than 1.0 during the peak hour. That is, the arriving demands are greater than the intersection can handle. Queues begin to form and grow while the demand to capacity (D/C) ratio is greater than 1.0. They only begin to dissipate when the D/C ratio becomes less than 1.0.

In Exhibit 2-43, we see the base case (Dataset 38) results for the PM Existing conditions. The delays range from 13.1 to 54.7 seconds per vehicle, the queues reach up to 10.9 vehicles on average and 20.0 vehicles at the 95th percentile, and the v/c ratios are 0.40 to 0.98. Five of the v/c ratios are 0.80 or above.
 
Exhibit 2-43. Clifton Country Road PM Existing Base Case Conditions
Condition Cycle
Length		 Performance Measure Eastbound Westbound Northbound Southbound OA
L T R Tot L T R Tot L T R Tot L T R Tot
Dataset 38
Base Case		 84.0 Delay 41.8 50.4 14.0 40.9 52.3 20.4 13.1 32.4 37.3 50.5 54.7 45.5 53.2 35.7 48.5 39.9
v/c ratio 0.40 0.97 0.45 - 0.92 0.69 - - 0.69 0.81 0.82 - 0.91 0.60 - -
95-Queue 1.8 20.0 11.4 - 16.7 18.9 - - 9.0 12.5 10.6 - 14.5 8.9 - -
Queue 0.9 10.9 5.9 - 9.0 10.3 - - 4.6 6.5 5.5 - 7.7 4.5 - -

The intersection is near capacity. Not all of the approaches have v/c ratios at or greater than 1.0, but a number of them have specific movements that are close. On some days, this intersection is over capacity. Demand probably does exceed capacity, and queues form. When we examine the PM With condition, we need to be prepared to check for D/C ratios greater than one and take appropriate actions.

  [ Back ] [ Continue ] to Sub-problem 4b