1. Lec 5: Capacity and LOS (Ch. 2, p.74-88)
Understand capacity is the heart of transportation issues.
Understand the fundamental flow of diagram
Understand how a shock wave is caused
Understand capacity analysis was set up for ideal cases and modifications are made to reflect prevailing conditions
Understand many factors, geometric, traffic, and control characteristics affect the capacity of a facility
Understand capacity and level of service correspond each other directly only for uninterrupted flow

2. Issues of traffic capacity analysis
How much traffic a given facility can accommodate?
Under what operating conditions can it accommodate that much traffic?
Highway Capacity Manual (HCM)
1950 HCM by the Bureau of Public Roads
1965 HCM by the TRB
1985 HCM by the TRB (Highway Capacity Software published)
1994 updates to 1985 HCM
1997 updates to 1994 HCM
2000 HCM is available

3. Highway capacity software

4. Flow-density relationships
Flow = (density) x (Space mean speed)
Space mean speed = (flow) x (Average space headway)
where
Average space headway = SMS/(Average time headway)
where

5. Speed, Density, and Flow Rate

6. A few examples…

7. Fundamental diagram of traffic flow (flow vs. density)
Optimal flow or capacity,qmax
Mean free speed, uf
Optimal speed, uo
Flow (q)
Speed is the slope. u = q/k
Uncongested flow
Congested flow
Jam density, kj
Optimal density, ko
Density (k)

8. Fundamental diagram of traffic flow (SMS vs. density & SMS vs. flow)uf uf
Uncongested flow
SMS
SMS
Congested flow kj
qmax
Density
Flow
SMS vs. density
SMS vs. flow

9. Fundamental diagram of traffic flow and shock wave
For upstream q1
Slope gives velocity uw of shock wave for q1
Flow (q) q2
Work zone
For bottleneck k2 k1 kj
Density (k)
Queue forms upstream of the bottleneck. So we use the diagram of the upstream section

10. HCM analyses are usually for the peak (worst) 15-min period.
Capacity concept
Capacity as defined by HCM:
“the maximum hourly rate at which persons or vehicles can be reasonably expected to traverse a point or uniform segment of a lane or roadway during a given time period under prevailing conditions.”
Sometimes using persons makes more sense, like transit
Some regularity expected (capacity is not a fixed value)
With different prevailing conditions, different capacity results.
Traffic
Roadway
Control

11. Capacity values for ideal conditions
Most capacity analysis models include the determination of capacity under ideal roadway, traffic, and control conditions, that is, after having taken into account adjustments for prevailing conditions.
Freeway, uninterrupted flow
12-ft lane width, 6-ft lateral clearance (right side, 2 ft on left side), all vehicles are passenger cars, familiar drivers, level grade, no heavy vehicles, free-flow speeds (70 mph for urban, 75 mph for rural; if you have a speed limit, it’s a different matter. Capacity used is usually average per lane (e.g pcphpl in one direction)

12. Prevailing condition types
Geometric conditions
Horizontal & vertical alignment, lane width and lateral clearance, grades
Traffic conditions
Directional distribution, lane distribution, heavy vehicles in the traffic stream, turning movements
Control factors
Speed limits, lane use controls, traffic signals, STOP and YIELD signs

13. Factors affecting: examples
Trucks occupy more space: length and gap
Drivers shy away from concrete barriers

14. From ideal conditions to real, prevailing conditions
We use adjustment factors to take into account the effect of prevailing conditions on capacity and level of service. Typically it is like…
Free-flow speed:
Passenger car equivalent flow rate:

15. Application of the capacity concept
Precision
Low
Used in transportation planning studies to assess the adequacy or sufficiency of existing highway networks (Traffic volumes are estimates; define targeted LOS first then find the number of lanes)
Medium
Used as a design control in the selection of highway type and in determining dimensional needs (Traffic volumes are estimates; define targeted LOS first then find the number of lanes)
High
Used in traffic operational analyses: a) Analysis of existing conditions, b) Estimation of operational improvements (For this analysis volume, geometry and control data exist)

16. Level of service
“A level of service is a letter designation that describes a range of operating conditions on a particular type of facility.”
LOS A (best)
LOS F (worst or system breakdown) A
Free flow B
Reasonably free flow C
Stable flow D
Approaching unstable flow E
Unstable flow F
Forced flow

17. MOE in 2000 HCM Uninterrupted Fwy: Basic sections Density (pc/mi/ln)
Fwy: Weaving areas
Fwy: Ramp junctions
Multilane highways
Two-lane highways
Percent-time spent following
Average upgrade speed
Interrupted
Signalized intersections
Approach delay (sec/veh)
Unsignalized intersections
Average total delay (sec/veh)
Arterials
Average travel speed
Transit
Load factor (pers/seat)
Pedestrians
Space (sq ft/ped)

18. LOS example: freeway basic sections
Basic freeway segments: Segments of the freeway that are outside of the influence area of ramps or weaving areas.
See Exhibit 23-3.

19. LOS – density – flow rate - speed

20. Level of Service: general descriptions

21. LOS for basic freeway segmentsD

22. LOS for basic freeway segments (cont.)

23. Density range (pc/mi/ln)
LOS examples near SLC
LOS B
LOS C or D
LOS A
Level of service
Density range (pc/mi/ln) A B C
16.1 – 24.0 D
24.1 – 32.0 E
32.1 – 45.0 F
> 45.0
LOS E or F

24. Objective of highway design
Create a highway of appropriate type with dimensional values and alignment characteristics such that the resulting design service flow rate is at least as great as the traffic flow rate during the peak 15-min period of the design hour, but not greater enough as to represent extravagance or waste
Why the peak 15-min period?  Traffic flow fluctuates, but it is known from previous studies that it is stable for about 15 minutes.

25. Service flow rates vs. service volumes
What is used for analysis is service flow rate. The actual number of vehicles that can be served during one peak hour is service volume. This reflects the peaking characteristic of traffic flow.
Stable flow
SFE
Unstable flow E F
Flow D C
SFA
SVi = SFi * PHF B A
Density

26. Design service flow rate vs. design volume
Design volume (DHV, vph)
Hourly volume of traffic estimated to use a certain type of facility during the design year (peak period)
Design service flow rate (vph)
Maximum hour flow rate of traffic that a new facility can serve without the degree of congestion falling below a pre-selected level

27. Acceptable degree of congestion
Rural freeway
Motorists expect high speed smooth traffic always
Urban arterial
Motorists accept few delays because they know there are physical limits for improvements (and budgets)
Balance need (demand) and resources available (supply) to determine the degree of congestion for design.

28. Targeted LOS