Module 2 : Traffic Measurement Procedures

Lecture 05 : Measurement at a Point

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Density method

In the density method, the PCU of truck ($E_t$) is computed as:

$$E_t = \frac{(k_c/W_l)}{(k_t)/W_l)}$$ (1)

where $k_c$ is the density of cars in pure homogeneous conditions(car/km.), $W_l$ is the width of the lane in homogeneous traffic, $k_t$ is the density of the truck in pure homogeneous conditions and $E_t$ is the passenger car unit of the trucks given homogeneous traffic behaviour. In density method where car following and lane discipline behaviour prevails, all traffic entities use an equal $W_l$.

Numerical Example

The table given below shows the data of flow and space mean speed of Car and HCV in a two lane road without shoulders. Assume the 85 percentile distribution width of HCV and Car to be 9.50m. and 7.50m. Compute the PCU value of HCV for each time interval.

Table 1: space mean speed of Car and HCV in a two lane road without shoulders

From	To	HCV Flow	HCV Mean Speed	CAR Flow	CAR Mean Speed
2.30	2.40	4	10.4	16	14.32
2.40	2.50	6	9.09	19	12.74
2.50	3.00	5	8.88	18	13.11
3.00	3.10	6	9.38	20	10.67
3.10	3.20	6	10.66	17	12.11
3.20	3.30	6	9.66	21	13.41
3.30	3.40	5	9.55	18	13.11
3.40	3.50	8	10.12	17	10.93
3.50	4.00	7	9.2	22	13.33
4.00	4.10	6	9.54	19	13.58
4.10	4.20	10	10.67	25	12.34
4.20	4.30	8	9.61	20	10.58

Solution

We know that PCU value can be calculated using the formula:

$$(PCU)_{truck} = \frac{(K_{car}/Wl)}{(K_{truck}/Wl)}$$ (2)

Step 1 Find the density of car and truck using basic relationship between the traffic flow parameters

$$Q = K\times V$$ (3)

Step 2 The using the method stated above we can find the PCU values. The table showing the PCU values has been illustrated below.

Table 2: PCU values for the above problem

From	To	HCV Flow	HCV Speed	Car Flow	Car Speed	Car density	PCU
2.30	2.40	4	10.4	27	14.32	1.86	3.68
2.40	2.50	6	9.09	32	12.74	2.49	2.86
2.50	3.00	5	8.88	30	13.11	2.29	3.09
3.00	3.10	6	9.38	33	10.67	3.12	3.71
3.10	3.20	6	10.66	28	12.11	2.34	3.16
3.20	3.30	6	9.66	35	13.41	2.61	3.19
3.30	3.40	5	9.55	30	13.11	2.29	3.32
3.40	3.50	8	10.12	28	10.93	2.59	2.49
3.50	4.00	7	9.2	37	13.33	2.75	2.75
4.00	4.10	6	9.54	32	13.58	2.33	2.82
4.10	4.20	10	10.67	42	12.34	3.38	2.74
4.20	4.30	8	9.61	33	10.58	3.15	2.88