Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter.
Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.
OCR for page 15
OPERATIONAL PRINCIPLES
Results from extensive field observations and data analyses have
demonstrated that ATL lane usage is governed by two primary factors: (1) the
through movement's degree-of-saturation on the subject approach, and (2) the
prevailing approach geometry.
The supporting data and evidence is presented in the following section.
Summary of Field Data
A total of 22 ATL approaches across the United States were selected for field
investigation and data collection based on the results of a web-based survey of
transportation practitioners. All study approaches add ATLs to the right of the
CTL and have a right-hand merge downstream. The sites have limited
obstructions to ATL use (e.g., driveways, transit stops, work zones, etc.) and
represent a range of geographic locations, geometric conditions, and levels of
congestion. Exhibit 3-2 displays a diagram of a typical ATL.
Exhibit 3-2
ATL Diagram
Exhibit 3-3 and Exhibit 3-4 list the 14 one-CTL and 8 two-CTL approaches
observed in this study. The two exhibits display location information, ATL
length components, whether the ATL is exclusive or shared, and the level of ATL
utilization by through traffic. ATL utilization in this context is defined as the
percentage of approach through traffic, calculated by dividing ATL through
volume by the total through volume on the approach.
As shown in Exhibits 3-3 and 3-4, nearly all ATLs are underutilized
compared to the lane utilization factors in the HCM 2010. According to the HCM
2010, the theoretical utilization should be 47.5 percent for an ATL with one CTL
and 31.7 percent for an ATL with two CTLs (2). This information confirms the
earlier statement regarding the need to consider the ATL as a separate lane
group.
Page 16
OCR for page 16
Down Average Exhibit 3-3
Upstream stream- Min. ATL ATL Max . ATL Summary of Study Approach
ATL ATL Utilization Utilization Utilization Characteristics for 1-CTL Sites
Length Length % % %
Approach Location Type (feet) (feet) Through Through Through
EB Walker at Murray Beaverton, OR Shared 570 150 21 28 35
WB Walker at Murray Beaverton, OR Shared 220 350 23 29 32
EB NC 54 at Fayetteville Durham, NC Exclusive 1650 450 19 23 27
NB La Canada at Magee Tucson, AZ Shared 780 430 15 19 25
SB La Canada at Magee Tucson, AZ Shared 580 720 11 18 27
EB Magee at La Canada Tucson, AZ Shared 620 390 14 19 25
WB Magee at La Canada Tucson, AZ Shared 700 500 9 14 19
NB La Canada at Orange Tucson, AZ Shared 640 590 11 19 25
Grove
SB La Canada at Orange Tucson, AZ Shared 730 560 18 19 24
Grove
EB Walker at 185th Beaverton, OR Exclusive 410 220 34 40 44
WB Walker at 185th Beaverton, OR Shared 350 310 13 15 17
SB Sunset Lake at Holly Holly Springs, Shared 420 950 3 9 13
Springs NC
NB Garrett at Old Chapel Hill Durham, NC Exclusive 320 300 13 19 24
SB Garrett at Old Chapel Hill Durham, NC Exclusive 330 380 15 23 27
EB = Eastbound, WB = Westbound, NB = Northbound, SB = Southbound
Down -
Exhibit 3-4
Upstream stream Min . ATL Average ATL Max . ATL Summary of Study Approach
Right - Turn ATL Length Length Utilization Utilization Utilization Characteristics for 2-CTL Sites
Approach Location Type (feet) (feet) % Through % Through % Through
NB MD 2 at Arnold Annapolis, MD Exclusive 800 300 15 19 22
SB MD 2 at Arnold Annapolis, MD Exclusive 1670 1060 13 20 31
EB MD 214 at Kettering Bowie, MD Exclusive 830 510 2 5 8
NB IL 171 at IL 64 Melrose Park, IL Shared 890 1000 13 18 24
SB IL 171 at IL 64 Melrose Park, IL Shared 1150 830 14 18 23
NB IL 171 at Roosevelt Melrose Park, IL Shared 290 230 4 6 9
SB IL 171 at Roosevelt Melrose Park, IL Exclusive 450 360 21 26 30
SB US 1 at New Falls of
Wake Forest, NC Exclusive 470 1040 11 13 15
Neuse
EB = Eastbound, WB = Westbound, NB = Northbound, SB = Southbound
Effect of Traffic Congestion on ATL Utilization
The primary motivation for a driver to use an ATL is to save travel time by
either avoiding long queues by moving around slower vehicles or avoiding
waiting at the light for more than one signal cycle (cycle failure). Thus, the level
of ATL use is controlled by operational elements of the intersection, including:
· Approach through-movement flow. Higher through flow rates on the
approach encourage more vehicles to move to the ATL. This result has
been confirmed in several general studies of short-lane use (7, 8, and 9).
· Signal timing. The lower the ratio of effective green for the approach to
intersection cycle length, the lower is the capacity of the approach.
Consequently, drivers are motivated to switch to the ATL to avoid a cycle
Page 17
OCR for page 17
failure. Additionally, a longer cycle length creates longer queues in the
CTL(s), which also encourages drivers to switch to the ATL.
· Arrival type. Most drivers choose to use the ATL while traffic is still
queued on the approach. Therefore, an ATL approach with a high
number of arrivals on green--usually achieved by good signal
progression--would experience lower ATL use than one with a random
arrival pattern.
· Right-turning vehicles and driveways. Driveways along either the
upstream or downstream length of the ATL create the potential for right-
turning vehicles to block the passage of ATL drivers, which discourages
ATL use. A heavy flow of right turns from a shared ATL has the same
effect, as shown in previous research (7, 8). Additional details on the
effects of right turns are provided in the ATL volume-estimation section
later in this chapter.
Exhibit 3-5 displays a plot of ATL through-movement flow rate against total
approach through-movement flow rate for all sites (each with a different
marker), broken down by the number of CTLs. All the indicated flow rates are
based on 15-minute counts expanded to an hourly rate. In the event that only
hourly volumes are available, they must first be divided by the peak-hour factor
(PHF) to yield the corresponding 15-minute peak flow rate.
The relationship shown in the exhibit between congestion and ATL use is
relatively strong, and more evident than a relationship between through traffic
and the percentage of ATL utilization, expressed as a fraction of all through
traffic. This latter relationship is weaker because ATL flow rate increases as the
overall through flow rate increases, making the ratio of the two more or less a
constant. For this reason, ATL flow in vehicles per hour (vph) was modeled
directly rather than predicted from percentage of utilization.
Page 18
OCR for page 18
Exhibit 3-5
1 CTL ATL Through-Movement Flow vs.
NB Garrett - E1
350 Total Through-Movement Flow
SB Garrett - E1
300 NB La Canada at Magee - S1
ATL Through-Movement Flow (vph)
SB La Canada at Magee - S1
250
EB Magee at La Canada - S1
200 WB Magee at La Canada - S1
NB La Canada at Orange Grove - S1
150 SB La Canada at Orange Grove - S1
NC 54 - E1
100
Sunset Lake - S1
50 EB Walker at 185 - E1
WB Walker at 185 - S1
0
0 100 200 300 400 500 600 700 800 900 1000 EB Walker at Murray - S1
Total Through-Movement Flow (vph) WB Walker at Murray - S1
2 CTLs
600 NB IL 171 at IL 64 - S2
SB IL 171 at IL 64 - S2
500
ATL Through-Movement Flow (vph)
NB IL 171 at Roosevelt - S2
400
SB IL 171 at Roosevelt - E2
300 NB MD 2 - E2
SB MD 2 - E2
200
MD 214 - E2
100
US 1 - E2
0
0 500 1000 1500 2000 2500 3000
Total Through-Movement Flow (vph)
EB = Eastbound, WB = Westbound, NB = Northbound, SB = Southbound
E1 = Exclusive, one CTL; S1 = Shared, one CTL; E2 = Exclusive, two CTLs; S2 = Shared, two CTLs
Many of the figures and tables in this report have been converted from color to grayscale for printing. The
electronic version of the report (posted on the web at www.trb.org) retains the color version.
Page 19
OCR for page 19
Exhibit 3-6 displays a plot of ATL through-movement flow against XT, the
level of through-movement congestion, with XT defined as the approach through
volume-to-capacity (v/c) ratio:
Equation 3-1
where:
VT = Through-movement demand flow rate on the ATL
approach,
N = Number of CTLs on the approach,
ST = Through-movement adjusted saturation flow rate per
lane,
g = Effective green time for the approach through movement,
and
C = Intersection cycle length.
Equation 3-1 computes the v/c ratio for the CTL s only because that factor was
found to be the primary motivator for using the ATL. Also, all calculations
should be based on 15-minute flow rates, including the average green and cycle
time, in the event that the traffic signal is actuated.
The relationships displayed in Exhibits 3-5 and 3-6 imply that separate
models are necessary to predict the ATL flow for one-CTL and two-CTL
approaches. A two-CTL site was omitted in the model development phase
because it exhibited different characteristics than the other sites. That site,
MD 214 (the data for which are circled at the bottom of Exhibit 3-5), had excellent
signal progression, which inhibited the use of the ATL, even though through
traffic flows were quite high. The remaining models should be considered valid
primarily under random arrival conditions and should be used with caution
under other circumstances.
In summary, ATLs are more likely to be used by drivers on intersection
approaches that operate near their through-movement capacity without an ATL
in place. Lane utilization is also affected by arrival type, with improved
progression inhibiting the use of the ATL due to limited queuing and delay on
the approach, and by the volume of right-turning movements at the intersection
or adjacent driveways.
Page 20
OCR for page 20
Exhibit 3-6
1 CTL ATL Flow vs. Level of Through-
NB Garrett - E1
Movement Congestion (XT)
350
SB Garrett - E1
NB La Canada at Magee - S1
300
SB La Canada at Magee - S1
ATL Through-Movement Flow (vph)
250 EB Magee at La Canada - S1
WB Magee at La Canada - S1
200
NB La Canada at Orange Grove - S1
150 SB La Canada at Orange Grove - S1
NC 54 - E1
100 Sunset Lake - S1
EB Walker at 185 - E1
50
WB Walker at 185 - S1
0 EB Walker at Murray - S1
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4
WB Walker at Murray - S1
XT
2 CTLs
600 NB IL 171 at IL 64 - S2
SB IL 171 at IL 64 - S2
500
ATL Through-Movement Flow (vph)
NB IL 171 at Roosevelt - S2
400
SB IL 171 at Roosevelt - E2
300
NB MD 2 - E2
200 SB MD 2 - E2
MD 214 - E2
100
US 1 - E2
0
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4
XT
EB = Eastbound, WB = Westbound, NB = Northbound, SB = Southbound
E1 = Exclusive, one CTL; S1 = Shared, one CTL; E2 = Exclusive, two CTLs; S2 = Shared, two CTLs
Page 21
OCR for page 21
Effect of ATL Geometry
The upstream storage length of the ATL should be long enough to
adequately contain the maximum expected (i.e., 95th percentile) queue in the
ATL. Ideally, it should also be longer than the maximum expected queue in the
adjacent CTL to ensure that drivers have access to the ATL.
The results of this research indicate that the downstream length of an ATL
has virtually no impact on the level of ATL use, in spite of several earlier studies
that hypothesized that a longer downstream length would encourage ATL use (7,
8, 9, 10, 11). Exhibit 3-7 displays a plot of the observed ATL utilization for each
study approach against the corresponding downstream length.
Exhibit 3-7
Minimum, Average, and 50
1 CTL
Maximum ATL Utilization vs.
Downstream Length 45
40
ATL Utilization (%)
35
30
25
Avg ATL %
20
15
10
5
0
0 100 200 300 400 500 600 700 800 900 1000
Downstream Length (ft)
2 CTLs
35
30
25
ATL Utilization (%)
20
15 Avg ATL %
10
5
0
0 200 400 600 800 1000 1200
Downstream Length (ft)
Page 22
