With the rapid growth of urbanisation in recent decades, urban transportation has become one of the major issues, and urban traffic congestion is the concern of mismatch cars with the development facilities of urban transportation. Density is created due to the competition between users of the limited capacity of the transportation system. This social phenomenon is accompanied with problems such as wasting time, wasting opportunities to live, and air pollution, and noise, waste of national resources such as fuel, mental health problems, and neurological disorders. Therefore, the identification and estimation of external costs caused by traffic congestion is important to attract the attention of researchers and policy-makers.

In this paper, overviews of studies on all the parameters that affect the congestion cost have been conducted. These parameters include the cost of traffic congestion, including vehicle operating costs, pollution, travel time and accidents. The different methods used to calculate any of these costs have been introduced and the method of calculating each parameter vector is presented. The principal developed relations for such calculation in this research are as following:

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C ^E _Con: External cost of congestion

α _S: Factor related to increasing costs due to neurological disorders

C ^E _Acc: External costs of congestion due to traffic accidents

C ^E _Delay: External costs of congestion due to wasted times and delays

C ^E _AP : External costs of congestion due to air pollution

VOC _Total: Additional external operational costs of vehicles

All costs are in Rial/Km. Vehicular operational costs are calculated based on the following relation:

VOC _Total = VOC_F + VOC_M + VOC_S

The elements of operational costs are fuel consumption, periodical services, and repairs. The relation for calculation of costs due to delays is as following:

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VOT _Total : The value of time for all trips in Rial/hour

T_CS = delay in comparison with the average speed in freeway (70 km/h)

β_P = factor related to vehicle occupation (person/veh)

V_CS = average of traffic volume (veh/h)

L_CS = total length of the freeway in this study (km)

The additional cost of pollution due to congestion is calculated as following:

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C ^E _Ap: Additional external cost of pollution

P: Factor of pollution dissipation in passenger cars

U ¹ _p: The cost of one metric ton of dissipated pollution in Iran

The additional cost of congestion due to traffic accidents, has been based on a comprehensive study of accident cost performed earlier in Iran.

Applying the abovementioned research method, and assuming 6 hours of congestion in Hemmat freeway in Tehran in a working day, the average total cost of congestion for such working day, has been computed for six different average speeds of 10 to 60 km/h. The cost related to average speed of 60 km/h is equal to 3 396 681 162 Rial per one kilometre, which is equal to 91802 US Dollar per kilometre.

Calculating the external costs of density with these number of elements that influence on it, and the data used relative to freeways in Tehran (Iran capital) are the specific features and innovations of this paper. The result of this paper is to provide a comprehensive and practical method for calculating the external costs of highway congestion, that with using this information, it is possible to provide the appropriate solutions to invest properly in order to solve this problem.