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Level of Service. Section 4.5. Level of Service: The Basics. Measure performance by density. Ratio of operating capacity to speed. Units: passenger cars per mile per lane (pc/mi/ln) Denoted by the letters A-F These letters represent the Level of Service (LOS) of the freeway.

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level of service

Level of Service

Section 4.5

level of service the basics
Level of Service: The Basics
  • Measure performance by density.
  • Ratio of operating capacity to speed.
  • Units: passenger cars per mile per lane (pc/mi/ln)
  • Denoted by the letters A-F
  • These letters represent the Level of Service (LOS) of the freeway.
level of service density ranges
Level of Service & Density Ranges

(Values taken from 2000 Highway Capacity Manual, Exhibit 23-2)

freeway base conditions

Minimum 6 foot right side clearance

2 foot left side clearance

12 foot lane width

5 lanes per direction

Freeway Base Conditions
freeway base conditions1
Freeway Base Conditions

Access spacing 2 miles or greater

freeway base conditions2

All Passenger Car Composition

  • Commuter Driver Population

Level Terrain, grades no greater than 2 percent

Freeway Base Conditions

We then take those base conditions and make adjustments to match the characteristics of our freeway segment!!


(P&P pg 153 EQ. 4.5.2)

Level of Service is determined by the density of traffic in a particular segment, estimated by the following equation:


D = density in pc/mi/ln

Vp =flow rate in pc/h/ln

S = free flow speed in mi/h

let s start at the top

Vp = Flow Rate (pc/hr/ln)

(P&P pg 153 EQ. 4.5.3)

Let’s Start at the Top
  • Where
    • V = hourly volume (veh/hr)
    • PHF = peak hour factor
    • N = number of lanes per direction (ln)
    • fHV = heavy vehicle factor
    • fDP = driver population factor
peak hour factor phf

(P&P pg 151 EQ. 4.5.1)

Peak Hour Factor (PHF)
  • Represents the variation of traffic flow within one hour.
  • Typical freeway range is 0.80 to 0.95
  • Based on local conditions.

The equation for PHF is:

  • Where
  • V = hourly volume
  • q = flow rate
  • Nt = number of vehicles counted during time period t
peak hour factor example


Peak Hour Factor Example

GIVEN: Vehicles are counted on a freeway in 5 minute intervals. The highest vehicle count for any one 5 minute segment was 250 vehicles. The total number of vehicles counted for the hour was 2600.

REQUIRED: Compute the Peak Hour Factor

The peak hour factor for this segment of freeway is 0.87

Note that this value is in the typical range for freeway PHF.

heavy vehicle factor

(P&P pg 154 EQ. 4.5.4)

Heavy Vehicle Factor
  • Heavy vehicles converted to passenger cars using the Heavy Vehicle Factor, fHV
  • Where
  • PT = proportion of trucks and buses in traffic
  • PR = proportion of recreational vehicles in traffic
  • ET = passenger-car equivalent for trucks and buses
  • ER = passenger car equivalent for recreational vehicles

These equivalent values can be found in HCM 2000, Exhibit 23-8 or P&P pg. 154

heavy vehicle factor example


Using the equivalent passenger-car values for level terrain:

The Heavy Vehicle Factor for this segment is 0.98

Heavy Vehicle Factor Example

GIVEN: From the previous PHF example, 2600 vehicles are counted on a level freeway segment, of which 90 were trucks & buses and 5 were recreational vehicles.

REQUIRED: Find the heavy vehicle factor for the hour.

Hint: Pay attention to the type of terrain.

driver population factor f dp
Driver Population Factor, fDP
  • Changes the flow rate based driver familiarity.
  • Ranges from 1.00 (Commuters) to 0.85 (Tourists and people unfamiliar with the facility).
  • Note: a driver population that is most familiar with the facility does not affect the rate of flow at all.
flow rate example


Using N = 3 and fDP = 1.00

Flow Rate Example

Now that we have all the parts, let’s calculate the flow rate of the freeway segment from our previous examples.

GIVEN: 6 lane freeway in an urban area, level terrain, 2600 vehicles counted in a given hour, and a commuter population.

Recall from previous examples that PHF = 0.87 and fHV = 0.98

REQUIRED: Compute the Flow Rate VP for the segment.

free flow speed

(P&P pg 154 EQ. 4.5.5)

Free-Flow Speed

The bottom of the LOS equation is the free flow speed (FFS) of the highway segment. Free flow speed is measured in miles per hour. The equation for FFS is:

  • Where
  • BFFS = base free flow speed, usually 70 mi/hr (urban) or 75 mi/hr (rural)
  • FLW = Factor for lane width shorter than 12 feet (HCM Exhibit 23-4)
  • FLC = factor for lateral (right side) clearance under 6 feet (HCM Exhibit 23-5)
  • FN = factor for number of lanes less than 5 lanes per direction (HCM Ex 23-6)
  • FID = factor for density of interchanges per mile (HCM Exhibit 23-7)
free flow speed example
Free Flow Speed Example

Continuing our previous example, let’s now calculate the free flow speed for the freeway segment.

GIVEN: Added information to previous example, the width of the lanes is 11 feet, the right-side clearance is 5 feet, and there are approximately 1.0 interchanges per mile.

Recall: Urban area, number of lanes N=3

Assume: BFFS = 70 mi/hr

REQUIRED: Calculate the base free-flow speed. Use values from the HCM 2000.

free flow speed example solution


The Free-Flow Speed for this segment, based on the local conditions, is 62.2 miles/hour.

Free Flow Speed Example Solution

Lane Width: 11 feet => FLW = 1.9 mi/h

Lateral Clearance: 5 feet, 3 lanes => FLC = 0.4 mi/h

Number of Lanes: 3 => FN = 3.0 mi/h

Interchanges: 1.0 per mile => FID = 2.5 mi/h

density calculation
Density Calculation

Now that we have all the parts, let’s calculate the density of our freeway segment.

GIVEN: Urban freeway segment.

Vp = 1016 pc/h/ln and S = 62.2 mi/h

REQUIRED: Compute the density and estimate the level of service.

density solution1

FFS = 70 mi/h

Flow Rate = 1016 pc/h/ln

Density Solution

The density for the segment is 16.33 pc/mi/ln. Referring to HCM 2000, Exhibit 23-2, this value corresponds with Level of Service B

los in action
LOS in Action

What does it all mean? Click the picture below to get a real life view of Level of Service A:

los in action ii
LOS in Action, II

Now click below to see a shot of LOS C in action: