Module 5 : Uninterrupted Flow
Lecture 24 : Freeway Operations
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Determination of LOS

A basic freeway segment can be characterized by three performance measures: density in terms of passenger cars per kilometer per lane, speed in terms of mean passenger-car speed, and volume-to-capacity (v/c) ratio. Each of these measures is an indication of how well traffic flow is being accommodated by the freeway. The measure used to provide an estimate of level of service is density. The three measures of speed, density, and flow or volume are interrelated. If values for two of these measures are known, the third can be computed.

Methodology

Level of service of an existing freeway is determined considering it as a stretch of basic freeway segment. It means that we have to take all the base conditions decided for basic freeway segment as a standard or initial input. The following steps are followed to determine the level of service of a freeway.
  1. The very first step of methodology is to collect all the input data like geometric data, measured FFS or BFFS, volume.
  2. volume adjustment: The hourly volume is converted into flow rate of passenger cars i.e pc/hr/ln.
  3. Computation of FFS: If BFFS is the input, then for getting the value of FFS ,we have to adjust the BFFS for the lane width,number of lanes,interchange density and lateral clearance.
  4. computation of S(average passenger car speed): S is calculated from the FFS. If FFS is measured directly in field, then FFS can be taken as S.
  5. Speed-flow curve is designed and speed is determined using this curve.
  6. Density is determined from the flow rate and speed taken from the speed-flow curve.
  7. Based on the density, the corresponding level of service(LOS) can be determined .
The steps involved in calculation of LOS are-
  1. Calculation of flow rate ($ V_p$)
  2. Calculation of average passenger car ($ S$)
  3. Calculation of density ($ D$) and determining LOS

Calculating Flow rate ($ V_p$)

The hourly flow rate must reflect the influence of heavy vehicles, the temporal variation of traffic flow over an hour, and the characteristics of the driver population. These effects are reflected by adjusting hourly volumes or estimates, typically reported in vehicles per hour (veh/h), to arrive at an equivalent passenger-car flow rate in passenger cars per hour (pc/h). The equivalent passenger-car flow rate is calculated using the heavy-vehicle and peak-hour adjustment factors and is reported on a per lane basis (pc/h/ln). The flow rate can be given as-

$\displaystyle V_p = \frac{V}{PHF \times N \times f_{HV} \times f_P}$ (1)

where, $ V$ = hourly volume, $ PHF$ = peak hour factor (0.80-0.95), $ N$ = no. of lanes, $ f_{HV}$ = heavy vehicle adjustment factor, $ f_P$ = driver population factor

Peak hour factor (PHF)

The peak-hour factor (PHF) represents the variation in traffic flow within an hour. Observations of traffic flow consistently indicate that the flow rates found in the peak 15-min period within an hour are not sustained throughout the entire hour.

$\displaystyle PHF = \frac{V}{V_{15\times4}}$ (2)

Where, $ V$ = hourly volume in veh/hr for hour of analysis, $ V_15$ = Maximum 15-min flow rate within peak hour, $ 4$ = number of 15-min period per hour.
On freeways, typical PHFs range from 0.80 to 0.95. Lower PHFs are characteristic of rural freeways or off-peak conditions. Higher factors are typical of urban and suburban peak-hour conditions. Field data should be used, if possible, to develop PHFs representative of local conditions.

Heavy vehicle adjustment factor ($ f_{HV}$)

Freeway traffic volumes that include a mix of vehicle types must be adjusted to an equivalent flow rate expressed in passenger cars per hour per lane. This adjustment is made using the factor $ f_{HV}$. Once the values of $ E_T$ and $ E_R$ are found, the adjustment factor, $ f_{HV}$, is determined by using equation given below -

$\displaystyle f_{HV} = {1}{1 + P_T(E_T -1) + P_R(E_R -1)}$ (3)

where, $ E_T$, $ E_R$ = passenger car equivalents for truck buses and recreational vehicles (RV's) in traffic stream respectively, $ P_T$, $ P_R$ = proportion of truck/buses and recreational vehicles in traffic stream. Adjustments for heavy vehicles in the traffic stream apply for three vehicle types: trucks, buses, and RVs. There is no evidence to indicate distinct differences in performance between trucks and buses on freeways, and therefore trucks and buses are treated identically. The factor $ f_{HV}$ is found using a two-step process. First, the passenger-car equivalent for each truck/bus and RV is found for the traffic and roadway conditions under study. These equivalence values, $ E_T$ and $ E_R$, represent the number of passenger cars that would use the same amount of freeway capacity as one truck/bus or RV, respectively, under prevailing roadway and traffic conditions. Second, using the values of $ E_T$ and $ E_R$ and the proportion of each type of vehicle in the traffic stream ($ P_T$ and $ P_R$), the adjustment factor $ f_{HV}$ is computed.

Driver population factor:

Under base conditions, the traffic stream is assumed to consist of regular weekday drivers and commuters.Such drivers have a high familiarity with the roadway and generally maneuver and respond to the maneuvers of other drivers in a safe and predictable fashion. But weekend drivers or recreational drivers are a problem. Such drivers can cause a significant reduction in roadway capacity relative to the base condition of having only familiar drivers. To account for the composition of the driver population, the fp adjustment factor is used and its recommended range is 0.85 – 1.00.