Module 2 : Traffic Measurement Procedures

Lecture 05 : Measurement at a Point

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Method based on headway

Realizing one of the primary effects of heavy vehicles in the traffic stream is that they take up more space, headways have been used for some of the most popular methods to calculate PCUs. In 1976, Werner and Morrall suggested that the headway method is best suited to determine PCUs on level terrain at low levels of service. The PCU is calculated as

$$E_t = \frac {(\frac{H_m}{H_b})- P_c}{P_t}$$ (1)

where $H_M$ is the average headway for a sample including all vehicle types, $H_B$ is the average headway for a sample of passenger cars only, $P_C$ is the proportion of cars, and $P_T$ is the proportion of trucks.

Numerical Example

The table given below show headway data for a number of traffic conditions. It is assumed that the traffic contains only car and truck. Compute the PCU value for each traffic condition Note that $h_m,~h_c,~p_c,~p_t$ respectively denote the average headway for mixed traffic, average headway for traffic consisting of cars only, the percentage of cars and percentage of trucks of the traffic stream.

Table 1: Headway data for a number of traffic conditions

$h_m$	$h_c$	$p_c$	$p_t$
2.70	2.5	0.90	0.10
2.80	2.5	0.85	0.15
2.94	2.5	0.80	0.20
3.10	2.5	0.75	0.25
3.25	2.5	0.70	0.30
3.35	2.5	0.65	0.35
3.70	2.5	0.50	0.50
3.80	2.5	0.45	0.55
3.95	2.5	0.40	0.60
4.20	2.5	0.30	0.70

Solution

Use the formula given above to find the value of PCU.

Table 2: Table for the value of PCU

$h_m$	$h_c$	$p_c$	$p_t$	$E_t$
2.70	2.5	0.90	0.10	1.80
2.80	2.5	0.85	0.15	1.80
2.94	2.5	0.80	0.20	1.88
3.10	2.5	0.75	0.25	1.96
3.25	2.5	0.70	0.30	2.00
3.35	2.5	0.65	0.35	1.97
3.70	2.5	0.50	0.50	1.96
3.80	2.5	0.45	0.55	1.95
3.95	2.5	0.40	0.60	1.97
4.20	2.5	0.30	0.70	1.97

Figure 1: Graph showing the variation of PCU with percentage of truck using the data of the problem given above