Cetb 412 - Capacity Los
Cetb 412 - Capacity Los
Cetb 412 - Capacity Los
TRANSPORTATION
ENGINEERING
C A PA C I T Y A N D L E V E L O F S E R V I C E
Chapter
1. Definitions
2. Level of Service (LOS)
3. Freeway Segment LOS Determination
a. Free-flow speed
b. Flow Rate
Freeway Definition
Number of lanes
Traffic composition
Lanes width
Driver population
Lateral obstruction
speed
Interchange spacing
Grade
Free-Flow Speed
■ Free-Flow Speed (FFS)
– The mean speed of passenger cars that can be
accommodated under low to moderate flow rates on a
uniform freeway segment under prevailing roadway and
traffic conditions.
LOS Calculation
Determine LOS
Speed-Flow-Density Relationship
Determining FFS
■ Measure FFS in the field
– Low to moderate traffic conditions
FFS BFFS f LW f LC f N f ID
FFS = free-flow speed (km/h)
BFFS = base free-flow speed, 110 km/h (urban), 120 km/h (rural)
fLW = adjustment for lane width (km/h)
fLC = adjustment for right-shoulder lateral clearance (km/h)
fN = adjustment for number of lanes (km/h)
fID = adjustment for interchange density (km/h)
Freeway LOS
USA
V
vp
PHF N f HV f p
V
PHF
V 15 4
■ Two-step process
– Determine passenger-car equivalents (ET)
– Determine fHV
Freeway LOS
Determine fHV
1
f HV
1 PT E T 1 P R E R 1
■ Two-step process
– Determine passenger-car equivalents (ET)
– Determine fHV
Freeway LOS
Determine Density
For 90 ≤ FFS ≤ 120 km/h AND (3100 – 15FFS) < vp ≤ (1800 + 5FFS)
v p 15 FFS 3100
2 .6
1
S FFS 23 FFS 1800
28 20 FFS 1300
S FFS
Determine LOS
Select a speed-flow curve based on FFS and flow rate
Determine LOS
Select density of flow
Determine LOS
Example 1
Existing 4-lane freeway, urban area with very restricted geometry.
Example 2
Example 3
v Rolling terrain
v 3.6m lane width
v 1.2m shoulder lateral clearance
v Interchanges spaced approximately 5 km apart.
v Traffic consists of 3% trucks, 7% buses and 8% recreational
vehicles.
v The adjustment for driver population factor is estimated to be 0.80.
v Peak 15 min flow rate is 1760 veh/hr