HomeMy WebLinkAbouttwo-lane highway reportCAPACITY Page 1 %W
TWO LANE HIGHWAYS
CAPACITY
Capacity =1, 700 pc/h for each direction, and 3,200 for both directions combined
The capacity of a two-lane highway is 1,700 pc/h (passenger cars per hour) for each direction of travel.
The capacity is nearly independent of the directional distribution of traffic on the facility, except that for
extended lengths of two-lane highway, the capacity will not exceed 3,200 pc/h for both directions of travel
combined. For short lengths of two-lane highway—such as tunnels or bridges—a capacity of 3,200 to
3,400 pc/h for both directions of travel combined may be attained but cannot be expected for an extended
length.
If the volume of traffic is greater than 1700 pc/h in one direction, or greater than 3200 pc/h in both
directions, then we are over capacity (LOS F) and no further analysis is needed to determine how the
facility is performing.
LEVEL OF SERVICE
The performance of a Class I two-lane highway in non -mountainous terrain and no traffic signals is
measured in relation to Average Travel Speed (ATS) and Percent Time Spent Following (PTSF). ATS
and PTSF are the Performance Criteria for 2 -Lane Highways.
Z
i
100
90
80
70
W
51)
40
20
10
4
EXHIBIT 20-3. LOS CRITMA (GRAPHICAL) FOR TWO-LANE HIC MAYS IN CLASS 1
O
A
A
30 35 40 45 5D 55 6D 65
Awap Ira* Speed (r ulh)
DETERMINING AVERAGE TRAVEL SPEED
The average travel speed (ATS) is estimated from the Free Flow Speed (FFS), the demand flow rate (Vp),
and an adjustment factor for the percentage of no -passing zones (np):
ATS = FFS - 0.4ai76vp -- f
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CAPACITY Page'$
v
Estimating FFS
The FFS can be estimated indirectly if field data are not available using the following equation, where
BFFS = Base Free Flow Speed, fLS = an adjustment factor for lane width and shoulder width, and fA = an
adjustment factor for number of access points per mile.
FFS = BFFS •-- fLS — fA
The design speed and posted speed limit of the facility may be considered in determining the BFFS. The
speed limit plus 5 mph or the design speed plus 5 mph are frequently used.
The first adjustment (fLS) relates to the effects of lane and shoulder widths on Free Flow Speed. Base
conditions for a two-lane highway require 12 -ft lane widths and 6 -ft shoulder widths.
EXHIB(f 20-5. ADJUSTANT A&) fOR LANE WIDTH ANU SHOLILCEit WIU1M
The second adjustment (fA) relates to the effect on Free Flow Speed of traffic entering and exiting
intersecting streets and driveways. The data indicates that each access point per mile decreases the
estimated FFS by about 0.25 mi/h. The access point density is found by dividing the total number of
intersections and driveways on both sides of the roadway segment by the length of the segment in miles.
An intersecting street or driveway should only be included if it influences traffic flow; access points with
little activity should not be included.
EXHB[t 20-6. ADJUSTMENT 04) FORA USS -POINT DENSITY
Aacros PLmndsper mi I Heduclr)n in -4 S (myii)
Ston rr FFS irnu'h;
StX3U b Of Wi dl Ih 1 R 1
2
2:2°-4
=4 a
-6
LjneWidth ffn
9 - 10
6.4
4.8
35
22
10 K 11
5.3
37
24
1.1
it 11 12
4.7
3A
1.7
Q4
z 124.Z
2-5
1.3
CLO
The second adjustment (fA) relates to the effect on Free Flow Speed of traffic entering and exiting
intersecting streets and driveways. The data indicates that each access point per mile decreases the
estimated FFS by about 0.25 mi/h. The access point density is found by dividing the total number of
intersections and driveways on both sides of the roadway segment by the length of the segment in miles.
An intersecting street or driveway should only be included if it influences traffic flow; access points with
little activity should not be included.
EXHB[t 20-6. ADJUSTMENT 04) FORA USS -POINT DENSITY
Aacros PLmndsper mi I Heduclr)n in -4 S (myii)
Determining Demand Flow Rate (Vp)
Three adjustments must be made to hourly demand volumes (V), whether based on traffic counts or
estimates, to arrive at the equivalent passenger -car flow rate (Vp) used in LOS analysis. These adjustments
are the PHF, the grade adjustment factor (fG), and the heavy vehicle adjustment factor (fHV). These
adjustments are applied using the following equation:
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20
5. El
30
7.5
40
1c) -c1
Determining Demand Flow Rate (Vp)
Three adjustments must be made to hourly demand volumes (V), whether based on traffic counts or
estimates, to arrive at the equivalent passenger -car flow rate (Vp) used in LOS analysis. These adjustments
are the PHF, the grade adjustment factor (fG), and the heavy vehicle adjustment factor (fHV). These
adjustments are applied using the following equation:
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CAPACITY
Grade Adjustment (fG)
Page $
3
The grade adjustment factor (fG) accounts for the effect of the terrain on travel speeds and percent time -
spent -following, even if no heavy vehicles are present.
Exm(T 20-7. GLADE ADJUSTMENT FACT OK (fG) TO DEI ERMINE SPEEDS ON TWO-WAY AND
DIRLCTION%. SEGMENTS
Rdrge of Tw.)-VVay Flava
Rinse of Direct ioml Flow
Type of
Ter rain
Leti%el
Raling
Rake (moi)
Rales tpc/
a 6W
0 600
0-300
1.00
0.71
600 12DO
> 30D 500
Wo
0-93
12001
J, 600
110
1 G.99
EXHIBIT20--8- GRADE AnJUS OEHT FACTOR OG) TO DETERMINE PERCENT TIME-SPENT-FOLLOWUNG OIN
TWO-WAY AND DIRECTIONAL SEGMENTS
Range of Two-way Flow
Range d Diredi,r*al Fiona
Tvve of Terrain
Level
kjlkr:„
Rales (PcJh )
tales [PGih)
a 6W
0 :IOU
1_M
0.77
> 600 1200
} 300 -600
1.00
094
> 1200
>600
1_00
1.€ 0
Adjustment for Heavy Vehicles (fHv)
The presence of heavy vehicles in the traffic stream decreases the FFS (Free Flow Speed) because, at base
conditions, the traffic stream is assumed to consist only of passenger cars. Therefore, traffic volumes must
be adjusted to an equivalent flow rate expressed in passenger cars per hour. This adjustment is
accomplished by using the factor fHV'
Adjustment for the presence of heavy vehicles in the traffic stream applies to two types of vehicles: trucks
and RVs. Buses should not be treated as a separate type of heavy vehicle but should be included with
trucks. The heavy -vehicle adjustment factor requires two steps. First, the passenger -car equivalency
factors for trucks (ET) and RVs (ER) for the prevailing operating conditions must be found. Then, using
these values, an adjustment factor must be computed to correct for all heavy vehicles in the traffic stream.
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CAPACITY
Page 01*
14
EXHIBIT 20-9. PASSENGER -CAR EQUIVALENTS FOR TRUCKS AND RVS TO DETERIANE SPEEDS ON
TWO-WAY AND DIRECTIONAL SEGMENTS
EXHIBIT 20-10. PASSENGER -CAR EQUIVALENTS FOR TRUCKS AND RVS TO DETERMit* PEFtCIENT
TIME -SPENT -FOLLOWING ON r%MO-WAY AND E)M TiDNAL SEGMENTS
Type of Terrain
Level
RiAling
Vehicle Type
Range of Tm-Wiv
Range ut Directional
Flow Males I w:A ;
Flom Reser, it)(YM
Trucks, E -r
0 600
U :SLxi
1.7
2-5
> 600 1,200
r 3W &UU
1.2
1.9
> 1,2W
> 6M
1.1
1.5
RVs. Ea
0 600)
a 3M
1.{l
1.1
6W 1.20U
} 3W WO
1.0
1.1
> 1.2m
> 9xl
1.0
1.1
EXHIBIT 20-10. PASSENGER -CAR EQUIVALENTS FOR TRUCKS AND RVS TO DETERMit* PEFtCIENT
TIME -SPENT -FOLLOWING ON r%MO-WAY AND E)M TiDNAL SEGMENTS
Once values for E.I. and ER have been determined, the adjustment factor for heavy vehicles (flit,) is
computed using the following equation where PT is the percentage of trucks in the traffic stream
(expressed as a decimal) and PR is the percentage of RV's in the traffic stream (also expressed as a
decimal):
fHV =
Iterative Computations for Vp
I
I+PRET--1)+PR(ER-1)
If the computed value of Vp is less than the upper limit of the selected flow -rate range for which fG, ET,
and ER were determined, then the computed value of Vp should be used. If the Vp is higher than the upper
limit of the selected flow -rate range, repeat the process for successively higher ranges until an acceptable
value of Vp is found. Because the highest range includes all flow rates greater thanl200 pc/h in both
directions of travel combined, it can be used if a computed value exceeds the upper limit of both lower
flow -rate ranges.
Estimating fnP
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Tyve ❑f T wrain
Level
Roiling
Vehicle Type
Range of Tm-Way
Range of Drecltnnal
Flow Rates {pc/n)
Flcmv Rates JI)cjh)
Truces, E_
U 6W
0 :i@:l
1.1
1,6
> tir.Xl 1,2DO
` .W ODO
1.1
1.5
1.200
> fA
1.0
1.0
RVs. ER
U 600
0 3417
1.0
1.0
r €M 1,2[10
300 600
1.0
1.0
} 1.;M
6T10
1.0
1.0
Once values for E.I. and ER have been determined, the adjustment factor for heavy vehicles (flit,) is
computed using the following equation where PT is the percentage of trucks in the traffic stream
(expressed as a decimal) and PR is the percentage of RV's in the traffic stream (also expressed as a
decimal):
fHV =
Iterative Computations for Vp
I
I+PRET--1)+PR(ER-1)
If the computed value of Vp is less than the upper limit of the selected flow -rate range for which fG, ET,
and ER were determined, then the computed value of Vp should be used. If the Vp is higher than the upper
limit of the selected flow -rate range, repeat the process for successively higher ranges until an acceptable
value of Vp is found. Because the highest range includes all flow rates greater thanl200 pc/h in both
directions of travel combined, it can be used if a computed value exceeds the upper limit of both lower
flow -rate ranges.
Estimating fnP
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CAPACITY
Page 8
Exl-*Hi[ 20-11. AOJUSTMENT (f„p) #-OR EFFECT OF NO-PAS5v4G ZCINES ONAVLR^GE TRAVEL SPEED 014
TWO-WAY SEGMENTS
Two- W ay Dertand Vic"
R21p• ti2. (pcAl
U
Z00
400
WU
sGo
l(K)o
12(]U
1400
lfk)o
18L)O
2mo
2."i
2400
2EOO
2800
3UQ0
:1X10
Reduction 1n A%pwr ge Travel Speed (mtitl)-
fe�Ls - fy :!. ti!►Y) ZU n8.5 {�lq a
0
20
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3-5
3.5
4.5
0.0
1 _ty
2.4
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3.4
S9
0 l
114
1.9
2-4
2.7
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1.6
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2.2
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0-8
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1.9
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1.2
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1.7
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EX"1311 20-12- ALLIUSTMENT (1 �j i -OK T;OKWINEL, EFhtGi Ot DlkkClIONAL DIS flUBIn DON01-
THAttlC AND PLW'tt4TACE llF Nt)-PI45SIN(: 2OWSON PLKCENT ON
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200
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114
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20,7
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to
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nai Sde 130/2o
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CAPACITY Pagea�Q%
DETERMINING PERCENT TIME -SPENT -FOLLOWING
The percent time -spent -following is estimated from the demand flow rate, the directional distribution of
traffic, and the percentage of no -passing zones.
PTSF = SPTSF + fa,,P
Estimating BPTSF
Base Percent Time Spent Following (BPTSF) is calculated using the formula:
BPTSF =100 I - e-0-000679vp'I
Vp is calculated exactly as before except that the exhibits pertaining to percent -time -following are used
instead of the exhibits pertaining to average travel speed to determine fG and fHV
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CAPACITY
b
Estimating fd/np
Page 9
1
Finally, an adjustment representing the combined effect of the directional distribution of traffic and the
percentage of no -passing zones is also made.
ExFBBfT 20-12- AWUSTMI:NT (f p FOR COMBINED EFFkCT OF DIRECTIONAL DISTRIBUTION of
TRAFFIC AND PERCENTAGE OF NO -PASSING ZONES ON PERCENT TIME-SPENT-fO LOWING ON
TWO WAYSEGMENTS
Doe eam[ Spit = 7(Y'30
s 2W
incrreaz-_ in PermN Tome -Spam I vWlawsn J041
Pico-PAsslnG Zones J%;
19.1
Tm-Way Flkm
0
20
40
60
80 1€ l
Rite, re [ 0CA, }
220
226
23.2
wo
0.0
LN redwruI
SPIN -50150
�= 200
0.0
10.1
17.2
201.2
21.0
21.8
400
0D
12.4
1910
22-7
Z3.8
24.8
10
OA
111
16.0
18.7
19.7
A is
800
0.0
9.0
1L3
14.1
14.5
15.4
1400
1.1.0
3,6
S.S
6 7
7.3
7.9
2000
()b
is
Z9
3.1
4,1
4.4
L%00
U D
1 1
1.6
2.9
2-3
Z4
:$2.j
0.0
0.7
C09
1.1
1.2
1 1.4
Ufed
im3l SPIN - 6W40
230
1-6
11A
11-2
225
23.1
23.7
400
05
11.7
16,2
20.7
21.5
222
6)00
9 Q
115
1S.2
1&9
19.8
20.7
80F:1
0.0
7.6
1 CI.3
13.0
13.7
14.4
1400
0.0
3.7
5.4
7,1
7.6
8.1
2CM
0A
23
14
36
4.0
4.3
. 26010
0:0 1
09
1.4
1.9
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Z2
Doe eam[ Spit = 7(Y'30
s 2W
2-A
13.4
19.1
14.8
25.2
25.5
400
1.1
125
17.3
220
226
23.2
wo
0.0
11.6
15,4
19.1
201.0
20-9
M0
()JO
7.7
10.5
13.E
14.0
14.6
140€3
0.0
3.8
5.6
7.4
7.9
& 3
Low
OA
1.4
4.9
3.5
s_ 9
4.2
Dretlso ml SpIII = 8CV20
200
5.1
17.5
24.3
31.0
31.3
31.6
400
25
15.6
21.5
17.1
Z7.6
28.0
600
0.0
14 D
18.6
23.2
219
24.5
800
0.0
93
1 Z 7
1 i'i.0
16.5
17.0
1400
0.0
4.6
fi. T
8.7
9.1
9.5
2000
OD
2A
3.4
4.5
4.7
4.9
DArediawl Spic-9CV10
5 2(1015b
21 b
'29.4
37.2
3T.4
37.6
404
24
19.0
25.6
32.2
3L.5
328
6010
0.0
163
21.8
21.2
27.6
28.0
9f3U
Uzi
10.9
14.8
1&6
19.0
19.4
?1400
0.0
5.5
7.8
10.0
10.4
1a7
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