1

Lec 5: Capacity and Level-of-Service Analysis for Freeways, Multilane Highways & 2-Lane 2-Way

Highways (p.2-60 to 2-70)

Explain why capacity is the heart of transportation issues. Define capacity and level-of-service concept and explain why

capacity is not a fixed value Explain the relationship between the v/c ratio and level of

service Estimate (determine) the free-flow speed of a freeway or a

multilane Obtain proper passenger-car equivalents for trucks, buses, and

RVs (Grade affects the performance of these vehicles) Conduct design and planning analyses for the basic freeway

and multilane highway segments (apply the knowledge of capacity and LOS to the redesign of Moark Junction.

Chapter objectives: By the end of these chapters the student will be able to:

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

2001 updates to 2000 HCM

2010 HCM

3

Highway Capacity Software

4

2.4.4 The capacity concept

The capacity of a facility is:

“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.”

Traffic

Roadway

Control

With different prevailing conditions, different capacity results.

HCM analyses are usually for the peak (worst) 15-min period.

Some regularity expected (capacity is not a fixed value)

Sometimes using persons makes more sense, like transit

5

2.4.5 Level of service

“Level of service (LOS) is a quality measure describing operational conditions within a traffic stream, generally in terms of such service measures as speed and travel time, freedom to maneuver, traffic interruptions, and comfort and convenience.”

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 or breakdown flow

SFA

SFB

SFC

SFD

SFE

Table 2-4, p. 2-66

6

MOE in 2010 HCMUninterrupted Fwy: Basic sections Density (pc/mi/ln)

Fwy: Weaving areas Density (pc/mi/ln)

Fwy: Ramp junctions Density (pc/mi/ln)

Multilane highways Density (pc/mi/ln)

Two-lane highways Percent-time spent following, Average travel speed, and Percent free-flow speed

Interrupted Signalized intersections

Approach delay (sec/veh), and v/c

Unsignalized intersections

Average total delay (sec/veh)

Arterials Average travel speed

Transit Load factor (pers/seat)

Pedestrians Space (sq ft/ped)

7

The v/c ratio and its use in capacity analysis

v/c = Rate of flowCapacity

The volume capacity ratio indicates the proportion of the facility’s capacity being utilized by current or projected traffic. Used as a measure of the sufficiency of existing or proposed capacity.

v/c is usually less than or equal to 1.0. However, if a projected rate of flow is used, it may become greater than 1.0. The actual v/c cannot be greater than 1.0 if departure volume is used for v.

A v/c ratio above 1.0 predicts that the planned design facility will fail! Queue will form.

The comparison of true demand flows to capacity is a principal objective of capacity and LOS analysis.

8

Freeways and multilane highways

Basic freeway segments: Segments of the freeway that are outside of the influence area of ramps or weaving areas.

Chapter 14 9

Basic freeway and multilane highway characteristics (This is Figure 14.2 for basic freeway segments, Roess,

Prassas, and McShane).

Equations for curves in Fig. 14.1

10

Table 14.1 of Roess, Prassas, McShane

11

(For multilane highways)

Base Speed-Flow Curves for Multilane Highways

Fig 14.3 of Roess, Prassas, McShane

12

Base Speed-Flow Curves for Multilane Highways

Fig 14.3 of Roess, Prassas, McShane

Chapter 14 13

2.4.5 Level of Service

LOS C or D

LOS B

LOS A

LOS E or F

Table 14.2 of Roess, Prassas, McShane

14

Service flow rates and capacity

Table 14.3 and Table 14.4 of Roess, Prassas, McShane

Chapter 14 15

Capacity and LOS analysis methodologies

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.

Multilane highways

12-ft lane width, 6-ft lateral clearance, all vehicles are passenger cars, familiar drivers, free-flow speeds >= 60 mph. Divided. Zero access points. Capacity used is usually average per lane (see slide 9)

Min. lane widths of 12 feet

Min. right-shoulder lateral clearance of 6 feet (median 2 ft)

Traffic stream consisting of passenger cars only

Ten or more lanes (in urban areas only)

Interchanges spaced every 2 miles or more

Level terrain, with grades no greater than 2%, length affects

Driver population dominated by regular and familiar users

Basic freeway segments

16

Prevailing condition types considered(p.291 of Roess, Prassas, and McShane):

Lane width

Lateral clearances

Type of median (multilane highways)

Frequency of interchanges (freeways) or access points (multilane highways)

Presence of heavy vehicles in the traffic stream

Driver populations dominated by occasional or unfamiliar users of a facility

17

Factors affecting: examples

Drivers shy away from concrete barriers

Trucks occupy more space: length and gap

18

Types of analysis

Operational analysis (Determine speed and flow rate, then density and LOS)

Service flow rate and service volume analysis (for desired LOS) MSF = Max service flow rate

Design analysis (Find the number of lanes needed to serve desired MSF)

pHVii

ii

pHVii

p

pHVp

ffMSFPHF

DDHVN

PHFSFSV

ffNMSFSF

S

vD

ffNPHF

Vv

***

*

***

***

19

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.

SVi = SFi * PHF

Stable flow

Unstable flow

Density

Flo

w

SFA

SFE

AB

C

D

E F

peakV

volumehourlyPeakPHF

_154

__

Congested

Uncongested

Chapter 14 20

Determining the free-flow speed (1)

AMLCLW

LCLW

ffffBFFSFFS

TRDffFFS

84.022.34.75

)/( pHVp ffNPHFVv

Free-flow speed (read carefully definitions of variables):

Passenger car equivalent flow rate:

Use either the graph or compute:

S

vD p

Then Table 14.2 for LOS.

See Figure 14.4 for multilane highway sections.

Basic freeway segments, eq. 14-5

Multilane highway sections, eq. 14-6

BFFS: • 60 mph without any data• Speed limit 40-45 mph, add 7 mph• Speed limit 50-55 mph, add 5 mph

Chapter 14 21

Adjustment to free-flow speed on a freeway

84.022.34.75 TRDffFFS LCLW

TRD = Total number of on- and off-ramps within ±3 miles of the midpoint of the study segment, divided by 6 miles.

Determining the free-flow speed (2)

22

Determining the free-flow speed (3)

Adjustment to free-flow speed on a multilane highway

AMLCLWi ffffBFFSFFS

fLW: use Table 14.5

Choosing a free-flow speed curve

Chapter 14 23

Not recommended to interpolate. So, this table was given. This table is for both freeways and multilane highways.

Chapter 14 24

Determining the heavy-vehicle factor

RRTTRT

RRTTP

RRTTHV

EPEPPP

EPEPP

EPEPf

11

1

1

1

)1()1(1

1

PP = percent passenger cars

PT = percent trucks & buses

PR = percent recreational vehicles (RVs)

ET = PCE for trucks and buses

ER = PCE for RVs

Grade and slope length affects the values of ET and ER.

pHVp ffNPHF

Vv

***

Chapter 14 25

How we deal with long, sustaining grades…

Extended segments

Type of Terrain

Level Rolling Mountains

ET (trucks & buses) 1.5 2.5 4.5

ER (RVs) 1.2 2.0 4.0

There are 3 ways to deal with long, sustaining grades: extended general freeway segments, specific upgrades, and specific downgrades.

(1) Extended segments: where no one grade of 3% or greater is longer than ¼ mi or where no one grade of less than 3% is longer than ½ mi. And for planning analysis.

26

How we deal with long, sustaining grades…(cont)

(2) Specific upgrades: Any freeway grade of more than ½ mi for grades less than 3% or ¼ mi for grades of 3% or more. (For a composite grade, refer to page 298 right column.) Use the tables for ET and ER for specific grades.

(3) Specific downgrades:

If the downgrade is not severe enough to cause trucks to shift into low gear, treat it as a level terrain segment, ET = 1.5.

Otherwise, use the table for downgrade ET

For RVs, downgrades may be treated as level terrain, ER = 1.2.

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Determining the driver population factor

Not well established Between a value of 1.00 for commuters to

0.85 as a lower limit for other driver populations

Usually 1.00 If there are many unfamiliar drivers use a

value between 1.00 and 0.85 For a future situation 0.85 is suggested

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Planning analysis

You want to find out how many lanes are needed for the targeted level of service.

Step 1: Find fHV using for ET and ER.

Step 2: Try 2 lanes in each direction, unless it is obvious that more lanes will be needed.

Step 3: Convert volume (vph) to flow rate (pcphpl), vp, for the current number of lanes in each direction.

Step 4: If vp exceeds capacity, add one lane in each direction and return to Step 2.

Step 5: Compute FFS.

Step 6: Determine the LOS for the freeway with the current number of lanes being considered. If the LOS is not good enough, add another lane and return to Step 3.

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Determining the driver population factor

Not well established Between a value of 1.00 for commuters to

0.85 as a lower limit for other driver populations

Usually 1.00 If there are many unfamiliar drivers use a

value between 1.00 and 0.85 For a future situation 0.85 is suggested

Lec 6: Two-Lane Highway: Classes

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• Class I highways: generally arterial highways that serve long-distance trips and on which motorists expect to travel at high speeds.

• Class II highways: highways that serve shorter trips and on which motorists do not expect to travel at high speeds.

• Class III highwqays: serve more developed areas

Two-Lane Highway: Design Standards & LOS

31

Table 16-1 and Table 16-4 from Roess, Prassas, and McShane 4th edition.

ATS = Average Travel Speed: Average speed of all vehicles traversing the defined analysis segment for the specified time period (peak 15 minutes)

PTSF = Percent Time Spent Following: Aggregate percentage of time that all drivers spend in queues, unable to pass, with the speed restricted by the queue leader. A surrogate measure for PTSF is the percentage of vehicles following others at headways of 3.0 seconds or less.

PFFS = Percent Free-Flow Speed: is based on the cmparisonof the prevailing speed to the free-flow speed, expressed in percentage.

Design Level of Service

32

Table 2-5 of GB2011, page 2-67.

AADT at the Moark Junction (2011)

33

ATR locations

34http://www.udot.utah.gov/main/uconowner.gf?n=200309160954472

Hour Begin US 6 EB US 6 WB US 6 Total EB/Total % US 6: % of ADT US89 SB US 89: % of ADT

0:00 68.0 63.7 131.7 51.6% 1.11% 6.3 0.60%

1:00 52.3 52.3 104.6 50.0% 0.88% 1 0.10%

2:00 49.0 38.7 87.7 55.9% 0.74% 0.7 0.07%

3:00 48.7 33.3 82.0 59.4% 0.69% 0.7 0.07%

4:00 53.3 41.0 94.3 56.5% 0.79% 4.3 0.41%

5:00 142.3 75.3 217.6 65.4% 1.83% 26.7 2.55%

6:00 329.0 156.3 485.3 67.8% 4.09% 29.3 2.79%

7:00 407.3 176.0 583.3 69.8% 4.91% 43 4.10%

8:00 510.0 231.4 741.4 68.8% 6.24% 61 5.82%

9:00 505.7 310.3 816.0 62.0% 6.87% 64 6.10%

10:00 494.0 299.5 793.5 62.3% 6.68% 57.2 5.45%

11:00 479.3 244.0 723.3 66.3% 6.09% 64 6.10%

12:00 480.3 220.3 700.6 68.6% 5.90% 61.5 5.86%

13:00 444.0 259.4 703.4 63.1% 5.92% 66.5 6.34%

14:00 486.0 272.7 758.7 64.1% 6.39% 68 6.48%

15:00 462.7 330.7 793.4 58.3% 6.68% 91 8.68%

16:00 465.7 283.7 749.4 62.1% 6.31% 78.7 7.50%

17:00 425.7 253.3 679.0 62.7% 5.72% 86.3 8.23%

18:00 403.7 275.0 678.7 59.5% 5.72% 70.3 6.70%

19:00 330.7 218.3 549.0 60.2% 4.62% 56.3 5.37%

20:00 259.7 219.3 479.0 54.2% 4.03% 50.3 4.80%

21:00 216.0 181.3 397.3 54.4% 3.35% 31 2.96%

22:00 159.7 147.3 307.0 52.0% 2.59% 17.3 1.65%

23:00 107.0 111.7 218.7 48.9% 1.84% 13.3 1.27%

Total 7380.1 4494.8 11874.9 1 1048.7 1

352009 data

Moark Junction Traffic Distribution

36

Hourly volumes at ATRs of UDOT can be found at:http://www.udot.utah.gov/main/f?p=100:pg:0::::T,V:3776,60913

How many lanes do these routes need?