Transcript Lec 5: Capacity and Level-of-Service Analysis for Highways (p.2-60 to 2-70)
Lec 5: Capacity and Level-of-Service Analysis for Freeways, Multilane Highways & 2-Lane 2-Way Highways (p.2-60 to 2-70)
Chapter objectives: By the end of these chapters the student will be able to: 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.
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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 2
Highway Capacity Software
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2.4.4 The capacity concept
The capacity of a facility is: HCM analyses are usually for the peak (worst) 15-min period.
“
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 Some regularity expected (capacity is not a fixed value) Sometimes using persons makes more sense, like transit With different prevailing conditions, different capacity results.
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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 Table 2-4, p. 2-66 SF A SF B SF C SF D SF E 5
MOE in 2010 HCM
Uninterrupted Fwy: Basic sections Fwy: Weaving areas Fwy: Ramp junctions Multilane highways Two-lane highways Interrupted Signalized intersections Unsignalized intersections Arterials Transit Pedestrians Density (pc/mi/ln) Density (pc/mi/ln) Density (pc/mi/ln) Density (pc/mi/ln) Percent-time spent following, Average travel speed, and Percent free flow speed Approach delay (sec/veh), and
v/c
Average total delay (sec/veh) Average travel speed Load factor (pers/seat) Space (sq ft/ped) 6
The v/c ratio and its use in capacity analysis
v/c = Rate of flow Capacity
The comparison of true demand flows to capacity is a principal objective of capacity and LOS analysis.
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.
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Freeways and multilane highways
Basic freeway segments: Segments of the freeway that are outside of the influence area of ramps or weaving areas.
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Basic freeway and multilane highway characteristics
(This is Figure 14.2 for basic freeway segments, Roess, Prassas, and McShane).
Chapter 14 9
Equations for curves in Fig. 14.1
Table 14.1 of Roess, Prassas, McShane 10
Base Speed-Flow Curves for Multilane Highways (For multilane highways) Fig 14.3 of Roess, Prassas, McShane 11
Base Speed-Flow Curves for Multilane Highways Fig 14.3 of Roess, Prassas, McShane 12
2.4.5 Level of Service
LOS B LOS A LOS C or D LOS E or F Chapter 14 Table 14.2 of Roess, Prassas, McShane 13
Service flow rates and capacity
Table 14.3 and Table 14.4 of Roess, Prassas, McShane 14
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)
Basic freeway segments
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 15
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 16
Factors affecting: examples
Trucks occupy more space: length and gap Drivers shy away from concrete barriers 17
Types of analysis
Operational analysis (Determine speed and flow rate, then density and LOS)
v p
V PHF
*
N
*
f HV
*
f p D
v p S
Service flow rate and service volume analysis (for desired LOS)
MSF = Max service flow rate SF SV i i
MSF i
*
N
*
SF i
*
PHF f HV
*
f p
Design analysis (Find the number of lanes needed to serve desired MSF)
N i
DDHV PHF
*
MSF i
*
f HV
*
f p
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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 SF E Unstable flow E F D SF A A B C Uncongested Density Congested
SV i = SF i * PHF PHF
Peak
_
hourly
_
volume
4
V
15 _
peak
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Determining the free-flow speed (1)
Free-flow speed (read carefully definitions of variables):
FFS FFS
75 .
4
BFFS f LW
f LW
f LC
f LC
3 .
22
TRD
0 .
84
f M
f A
Basic freeway segments, eq. 14-5 Multilane highway sections, eq. 14-6 Passenger car equivalent flow rate:
v p
V
/(
PHF
N
f HV
f p
) Chapter 14 • • BFFS: 60 mph without any data Speed limit 40-45 mph, • add 7 mph Speed limit 50-55 mph, add 5 mph Use either the graph or compute:
D
v p S
Then Table 14.2 for LOS.
20 See Figure 14.4 for multilane highway sections.
Determining the free-flow speed (2) Adjustment to free flow speed on a freeway
FFS
75 .
4
f LW
f LC
3 .
22
TRD
0 .
84
TRD
= Total number of on and off-ramps within ±3 miles of the midpoint of the study segment, divided by 6 miles.
Chapter 14 21
Determining the free-flow speed (3)
Adjustment to free flow speed on a multilane highway
FFS
BFFS i
f LW
f LC
f M
f A f LW
: use Table 14.5
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Choosing a free-flow speed curve
Not recommended to interpolate. So, this table was given. This table is for both freeways and multilane highways.
Chapter 14 23
Determining the heavy-vehicle factor
f HV
1
P T
(
E T
1 1 )
P R
(
E R
1 ) 1
P P
1
P T E T
P R E R v p
V PHF
*
N
*
f HV
*
f p
1
P T
P R
1 1
P T E T
P R E R P P
= percent passenger cars
P T
= percent trucks & buses
P R
= percent recreational vehicles (RVs)
E T
= PCE for trucks and buses
E R
= PCE for RVs Chapter 14 Grade and slope length affects the values of E
T
and E
R
.
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How we deal with long, sustaining grades…
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.
Extended segments E T (trucks & buses) E R (RVs) Level 1.5
1.2
Type of Terrain Rolling 2.5
Mountains 4.5
Chapter 14
2.0
4.0
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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 E
T
and E
R
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, E
T =
1.5.
Otherwise, use the table for downgrade E
T
For RVs, downgrades may be treated as level terrain, E
R
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 27
Planning analysis You want to find out how many lanes are needed for the targeted level of service.
Step 1 : Find f HV using for E T and E R .
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),
v p
, for the current number of lanes in each direction.
Step 4 : If
v p
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 29
Lec 6: Two-Lane Highway: Classes
• 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 30
Two-Lane Highway: Design Standards & LOS
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.
Table 16-1 and Table 16-4 from Roess, Prassas, and McShane 4 th edition.
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Design Level of Service
Table 2-5 of GB2011, page 2-67.
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AADT at the Moark Junction (2011)
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ATR locations
http://www.udot.utah.gov/main/uconowner.gf?n=200309160954472 34
Hour Begin 0:00 1:00 2:00 3:00 4:00 5:00 6:00 7:00 8:00 9:00 10:00 11:00 12:00 13:00 14:00 15:00 16:00 17:00 18:00 19:00 20:00 21:00 22:00 23:00 Total US 6 EB 68.0
52.3
49.0
48.7
53.3
142.3
329.0
407.3
510.0
505.7
494.0
479.3
480.3
444.0
486.0
462.7
465.7
425.7
403.7
330.7
259.7
216.0
159.7
107.0
7380.1
US 6 WB 63.7
52.3
38.7
33.3
41.0
75.3
156.3
176.0
231.4
310.3
299.5
244.0
220.3
259.4
272.7
330.7
283.7
253.3
275.0
218.3
219.3
181.3
147.3
111.7
4494.8
US 6 Total 793.4
749.4
679.0
678.7
549.0
479.0
397.3
307.0
218.7
11874.9
131.7
104.6
87.7
82.0
94.3
217.6
485.3
583.3
741.4
816.0
793.5
723.3
700.6
703.4
758.7
EB/Total % 51.6% 50.0% 55.9% 59.4% 56.5% 65.4% 67.8% 69.8% 68.8% 62.0% 62.3% 66.3% 68.6% 63.1% 64.1% 58.3% 62.1% 62.7% 59.5% 60.2% 54.2% 54.4% 52.0% 48.9% US 6: % of ADT 1.11% 0.88% 0.74% 0.69% 0.79% 1.83% 4.09% 4.91% 6.24% 6.87% 6.68% 6.09% 5.90% 5.92% 6.39% 6.68% 6.31% 5.72% 5.72% 4.62% 4.03% 3.35% 2.59% 1.84% 1 US89 SB 6.3
1 0.7
0.7
4.3
26.7
29.3
43 61 64 57.2
64 61.5
66.5
68 91 78.7
86.3
70.3
56.3
50.3
31 17.3
13.3
1048.7
US 89: % of ADT 0.60% 0.10% 0.07% 0.07% 0.41% 2.55% 2.79% 4.10% 5.82% 6.10% 5.45% 6.10% 5.86% 6.34% 6.48% 8.68% 7.50% 8.23% 6.70% 5.37% 4.80% 2.96% 1.65% 1.27% 1 2009 data 35
Moark Junction Traffic Distribution
10,00% 9,00% 8,00% 7,00% 6,00% 5,00% 4,00% 3,00% 2,00% 1,00% 0,00% US 6: % of ADT US 89: % of ADT 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?
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