Applicability of AHS Service for Traffic Congestion in Sag
Sections
Harutoshi YAMADA*1 ,Hiroyuki OUCHI*1 , Takayuki HIRASAWA*1 ,Kousuke YAMADA*1,
Yasunori KATAYAMA*2 , Ryota HORIGUCHI*3 , Masanori OIKAWA*4*1

The National Institute for Land and Infrastructure Management,
Ministry of Land, Infrastructure and Transport, Japan
1 Asahi, Tsukuba City, Ibaraki Prefecture 305-0804 Japan

Tel: (+81) 29 864 4496,

Fax: (+81) 29 864 0565

E-mail: yamada-h92q9@nilim.go.jp, oouchi-h92qv@nilim.go.jp, hirasawa-t92ta@nilim.go.jp,
yamada-k924a@nilim.go.jp*2

Hitachi Laboratory, AHS Research Associationth

11 floor, Sumitomo Fudosan Hibiya Building, 8-6 Nishi-Shimbashi 2-chome,
Minato-ku, Tokyo 105-0003 Japan
Tel: (+81) 3 3504 0505,

Fax: (+81) 3 3504 2727

E-mail: yasunori_katayama@pis.hitachi.co.jp*3i-Transport Lab. Co., Ltd.

5th floor, Dai2-Sansei Building, 3-7 Kanda Surugadai, Chiyoda-ku, Tokyo 101-0062 Japan
Tel: (+81) 3 5283 8527,

Fax: (+81) 3 5283 8528

E-mail: horiguchi@i-transportlab.jp*4

AHS Research Association

E-mail: oikawa@ahsra.or.jp

Abstract
Sag sections and tunnels cause 35% of the congestion on expressways in Japan. One of thefactors in congestion is imbalance in traffic lane utilization such as traffic concentration in thepassing lane and leeway in the cruising lane. Therefore the National Institute for Land andInfrastructure Management (NILIM) and the Advanced Cruise-Assist Highway System
Research Association (AHSRA) have been studying AHS services that would optimize thetraffic lane utilization rates by providing drivers with information before congestion occurs.
This paper will report on our analysis of actual traffic conditions at a congestion location in asag section (in the Tomei(Tokyo-Nagoya) Expressway). It will also report on the results fromexamination of the possibility of providing service, the algorithms for determining the timing
for service.

1

KEYWORDS
AHS(Advanced Cruise-Assist Highway Systems), Sag, Highway, Congestion
1. An overview of congestion in sag sections and tunnels
The loss from congestion on expressways is approximately 940 billion yen per year.
Congestion occurs mainly at toll gates, followed by sag sections and tunnels (Figure 1).

50

Others8%
Merging
Toll gates
lanes
36%
21%
Sag and tunnels
35%

40

Reduced by ETC
30

As an upcoming
issue

20
10

Source: 2003 ITS Handbook

Figure1. Breakdown of congestion incidence
on expressways by road structure0

The implementation of ETC in toll gates
and the widespread adoption of on-board
Figure2. Reduction of congestion due to increasingly
unit have been reducing toll gate
widespread use of ETC (toll gates on 18
Metropolitan Expressway lines)
congestion on an annual basis in Japan(Figure 2). At present, therefore, theresolution of congestion in sag sections and tunnels is the next issue.
2. Mechanism of congestion in sag sections
Sag sections are considered to be traffic capacity bottlenecks. The causal factors in congestion
occurring here are considered to be as follows:
As the vehicles group reaches the sag section( i ) Vehicle behavior in the1) Drivers do not notice5) Vehicles several
the change in grade
places back may stop
3)
Intervals
betweendirection of travel:
vehicles become short
50km/h
60km/h
70km/hCareless driving leads
6) Next vehicle group
80km/h
90km/hinitiates complete congestion
4)
Drivers
lower
their
speed
drivers to unconscious
and try to keep the intervals
Gentle upward2) Speed of leadslope
reductions of speed in
vehicles are reduced(100 ⇒90Km/h)sag sections, so that theCruising lanes have leeway because of
imbalance in traffic lane utilizationinterval between
vehicles becomes
excessively long. This
Vehicle group forms due to behavior
interferes with theCareless driving: Time headwayof specific vehicles
increases on upward slope(ex; vehicle at low speed)
efficient utilization of
Figure3. Basic mechanism of congestion in sag sections

2the traffic capacity of individual traffic lanes.
(ii ) Traffic lane utilization:
When vehicles concentrate excessively in the passing lane, it makes congestion that begins
in the passing lane. At that time, there is leeway in the cruising lane so that the traffic
capacity as a whole is not utilized efficiently.
(iii) Behavior of specific vehicles:
When a group of vehicles arrives at the sag section, the reduction in speed by the leading
vehicle creates a speed reduction shock wave. When another, larger vehicle group of
greater density arrives after it, then the next vehicle group catches up with the front and theshock wave is causing congestion. The sizes of the vehicle groups are thought to be
determined by whether or not they include causal vehicles that want to travel at low speed,or that have poor acceleration performance.(1)3. Congestion countermeasures
The essentials ofTable1. Congestion causal factors and countermeasurescountermeasures against
Focusing point for
Problems in Sag Sections
Countermeasures(Services)
improvement
congestion in sag sectionsi) Decreased capacity Urge the drivers to accelerate (1) Service to prevent speed reduction
can be summarized as
Current
due to carelessproblem
driving
Reduce time headway(2) Service to recover speed
follows: (i) Eliminate(3) Service to adjust traffic lanecauses of reductions inutilization rateii) imbalance in traffic
Adjust the balance of trafficlaneutilizationlane
utilization(4) Service to adjust traffic lane
speed in bottleneck
Futureutilization rate by use of road shoulder
problem
sections;iii) Inefficient lane
Induce leading vehicle in(5) Service to prevent the formation ofutilization due tovehicle group to change to thevehicle groups(ii) Adjust the balance ofvehicle groups
cruising lane
Object of short-term
Object of medium to long term
traffic lane utilization
JH testing complete
development
development
rates in advance; and (iii)
Avoid forming vehicle groups. When these are achieved, it is possible to utilize road sections
to the maximum extent of their innate traffic capacity.
We propose various services shown in Table 1 “(1) to (5)”. The provision of information todrivers is a method that can be realized at low cost without changing road structure.
The Japan Highway Public Corporation (JH) proposed the above services (1) and (2) for use
in sag sections of the Tomei Expressway in the Yamato district from 2003. These are warningsof speed reductions by means of message signs installed in sag sections, and the provision of
information to encourage the recovery of speed.(2) The JH reports that this information
provision has brought a 7% improvement in the traffic capacity when there is congestion.
Conventional congestion countermeasures are mainly focusing on after congestion
phenomena The AHS services such as items (3) traffic lane utilization rate adjusting serviceand (5) service to prevent the formation of vehicle groups, however, are new countermeasuresintended to reduce congestion by means of information provision before congestion occurs.
Here the roadside systems obtain a comprehensive view of overall traffic flow and provide

3drivers appropriate caution. These methods can be expected to realize an overall harmonious
traffic flow and so to have the effect of reducing congestion.(3) An overview of the services ispresented as follows.
○Traffic lane utilization rateadjusting service- An approach in terms of theefficient utilization of
traffic capacity, this isprovision of information
with the aim of achieving
an appropriate traffic lane
utilization rate immediately
before congestion occurs

Sensors
Vehicle-to-Road
Communications

On-board unit

Remain in
cruisinglane

The passing lane is
crowded. The cruising lane
has leeway.

Figure 4. Conceptual image of sag section congestion countermeasures(Figure 4).
○Service to prevent the formation of vehicle groups- This service urges appropriate caution by the drivers of certain vehicles with the aim ofpreventing the formation of vehicle groups, which are likely to trigger congestion, or
eliminating such groups.
4. Analysis of sag section traffic conditions
4.1. Traffic lane utilization conditions in sag sections
We observed Traffic lane utilization
conditions in sag sections of theTomei Expressway in the Yamato
district (three traffic lanes on one side,
see Figure 5) in order to study thefeasibility of a traffic lane utilization
Figure5. Longitudinal slope and measurement points in therate adjusting service.
Yokohama-Machida Interchange

Traffic
counter
24.03KP

Camera 2
22.03KP

0.51%

TowardAtsugiInterchange

Camera 1
21.50KP
Traffic
counter
21.52KP

2.38%

K1 sensor
20.80KP

1.87%

Traffic
counter
19.5KP

2.41%

2.37%

Yamato Tunnel

0.23%

0.40%

25KP

0.52%

Bottleneck(22.03KP)

24KP

23KP

22KP

21KP

20KP

Yamato district

Traffic volume and speed by lane ata point approximately 500 m backward of the sag section (Figure 6) are obtained from vehicle
sensor (traffic counter) data, video cameras newly installed on the roadside, and motion pathdata acquired using an AHS image processing sensor(K-1 sensor).
Immediately before congestion (State 3), the 1st cruising lane (CL1) has less traffic than thepassing lane(PL) or the 2nd cruising lane (CL2) by 75–100 PCU/5 minutes. Subsequently, theoccurrence of congestion in PL and CL2 is followed by CL1 also making a transition all at
once to a condition of congestion (State 4). The difference in traffic volume between CL1 andCL2 immediately before congestion is approximately 10% of the spot traffic volume. It meansthat at least 10% of the whole traffic capacity remains unutilized. Consequently, balancing of

4

4.2. Possibilities of lanechanging
We introduced an indicator

Traffic volume per lane

traffic lane utilization rates
will make it possible toincrease traffic capacity inthe bottleneck section.

Leeway in
CL1

Point speed (CL1)
Traffic volume (CL1)called “the lane change
possibility rate” for the

Point speed (CL2)Traffic volume (CL2)

Point speed (PL)Traffic volume (PL)Figure6. Traffic volume and speed in the Yamato district during
typical congestion

purpose of microanalysis ofcases in which drivers areencouraged to return to a

State 0: Traffic is flowing freely, with less traffic in PL than in otherlanes.
State 1: Traffic is flowing freely, with more traffic in PL than in CL1State 2: Traffic is flowing freely, with more traffic in PL than in CL2.
State 3: Critical state immediately before congestion
State 4: State after congestion has occurredcruising lane. The lanechange possibility rate is the
expected value for the
probability that a vehicle canfind a headway (time interval between vehicles) in a neighboring lane that allows changing
lanes while the vehicle travels a unit distance. Here this is calculated using the following
formula for each traffic lane based on motion path data for individual vehicles.

Pg i +1,i = ∑j

Lg i +1,i , j
LThe quantities here are:Pgi+1, i Lane change possibility rate for change from traffic lane i+1 to i
Lgi+1, i, j: Distance traveled side by side by Vehicle j with headway inwhich lane change is possible
L: Section length

Figure 7 explains the concept of this indicator. Assume that a single vehicle travels for one
kilometer in CL2, and at point (1) the driver sees a headway in neighboring CL1. That statecontinues until reaching point (2), a distance of 700 m. The value that expresses the degree of
possibility that this vehicle could change lanes from CL2 to CL1 (the ease of lane change) can
be calculated as follows:[Distance traveled side by side with headway]÷[Section length]＝700÷1000=0.7
This indicator also expresses how
far before the bottleneck portion
service must be provided in order forthe driver to be able to change lanes.

PL
CL2
CL1

Assume, for example, that a
traveling distance of 350 m isrequired as an absolute minimum for

Figure7. Concept of the lane change possibility rate5

lane change. Then under conditions where the lane change possibility rate is 70% in thesection before the bottleneck, providing service a distance of 350÷0.7=500 m in advance willassure that the driver has the leeway needed to make it possible to change lanes.
In order to calculate the lane change
Cumulative frequency distribution of headway between
possibility rate, it is necessary toleading and following vehicles during lane changes
Tomei Expressway outward bound side, 20.80 km point,
September 25, 2004 (K-1 sensor analysis results)
ascertain the time headway for100%
possible lane change. The
80%
60%
distribution of headway between
40%
leading and following vehicles
20%0%during lane changes in the specified
0
1
2
3
4
5
6
7
8
9
10
Headway time between vehicles during lane change (seconds)
section was determined for that
Figure8. Cumulative frequency distribution of headway between
purpose by analysis of video data
leading and following vehicles during lane changes(Figure 8). There was a headway of
three seconds or more in approximately 50% of the cases (an average of 3.18 seconds). Whenthe headway was less than three seconds, lane change occurs mainly after the speed is reduced
after congestion occurred. It is hypothesized from above that the absolute minimum headway
needed to make it possible to change lanes in conditions up to the point immediately before
congestion is three seconds.
The lane change possibility rate calculated by the above method is merged with the traffic
volume and speed by traffic lane at the same location as before (Figure 9). When congestion
occurs (at 6:35), the lane change possibility rate for a change from CL2 to CL1 diminishes

Traffic volume per lane(PCU/5 minutes)/Point speed per lane(km/h)

Lane change possibility rate
(CL2 to CL1)
Lane change possibility rate
(PL to CL2)
Traffic volume (CL1)
Traffic volume (PL)
Point speed (CL2)

Lane change possibility rate
(CL2 to PL)
Lane change possibility rate
(CL1 to CL2)
Traffic volume (CL2)
Point speed (PL)
Point speed (CL1)

tilb
s
o
p
g
h
c
n
L
e
ra
y

Figure9. Relationship between the lane change possibility rate and traffic volume and speed(Data for September 18, 2004)

6

drastically from 0.6 to 0.1. This fact gives an indication that lane change is not possible duringcongestion. According to the traffic states described earlier, the lane change possibility rate is
fully high enough in States 0 to 2. It is also apparent that the lane change possibility rate forchanges from CL2 to CL1 remains at a relatively high value in State 3 (critical state) up to thepoint before congestion occurs.
5. Algorithms for determining the timing for provision of service
The traffic lane utilization rate adjusting service conducts realtime information provision inaccordance with traffic conditions. Therefore, it would be ideal to predict the timing foroccurrence of congestion and to provide the service only at that time. Consequently, we
studied algorithms for determining the timing for provision of services based on traffic
counter data.
It is difficult to predict the exact timing for occurrence of congestion itself using macro
indicators such as average speed and traffic volume per five minutes that can be collected bymeans of traffic counter data.(3) In constructing the algorithm for determining the timing ofservice provision, therefore, our approach is not to take the accurate prediction of the timing
for occurrence of congestion. The proposal here is rather to take an approach founded on theassumption that when traffic conditions are such that congestion appears likely to occur withmore than a certain degree of certainty, then information is to be provided in a form that
accounts for the risk that congestion may not occur.
The criteria for determining State 2 and State 3, which are conditions immediately before
congestion occurs, are defined below. We also analyzed Statistical data to verify theappropriateness of timing the provision of service to occur when traffic flow undergoes atransition to State 2 or State 3. The results of that analysis are described in the following.
Figure 10 shows theaverage speed andtraffic volume per fiveFree Flow
minutes by traffic laneaccording to the traffic
counter at the location
noted above (21.52km
post), for all days in
CL1: Q(100, 180), V(55, 85)
CL2: Q(150, 210), V(70,100)
2003 when congestion
PL : Q(180, 240), V(70,100)
occurred in the Yamato
CL1: Q(70, 130), V(70, 100)sag section, plotted on a
CL2: Q(140, 200), V(85,115)
PL : Q(180, 250), V(85,115)
traffic volume andspeed (Q-V) plane. The
time zones are

Figure10. Relationship between traffic conditions and window frame

7

classified on the basis of the times at which congestion occurred on the days in question (plots
90-60 minutes before congestions occur are gray, 60-30 minutes before are purple, 30-0
minutes before are blue, 0-30 minutes after congestions occur are orange, 30-60 minutes after
are red). It becomes apparent that the ranges plotted are generally determined by the timezones before congestion. This suggests that the transitions between traffic conditions (State 0
to State 3) up to the time when congestion occurs generally follow the same pattern.
State 2 and State 3, which are timings for service provision, are also represented in Figure
10 by frame. The continuous times of service provision and whether or not congestion actuallyoccurred after service was provided were statistically distributed, and the following findings
were obtained as a result.
① After traffic conditions underwent a transition to State 2, those conditions weremaintained for an average of 17 minutes. If State 2 had been used for the timing of service
provision, the hit ratio for service provision with respect to occurrence of congestion
would have been 71%, while the missdetermination rate would have been 70%.
② After traffic conditions underwent a transition to State 3, those conditions weremaintained for an average of 14 minutes. If State 3 had been used for the timing of service
provision, the hit ratio for service provision with respect to occurrence of congestion
would have been 70%, while the missdetermination rate would have been 32%.
Here these terms are defined as follows:
Hit ratio＝(Number of times service was provided) / (Number of times congestion occurred)
Missdetermination rate＝(Number of times congestion did not occur after service wasprovided) / (Number of times service was provided)
The missdetermination rates were not low whether State 2 or State 3 was used for the timingof service provision. When traffic counter data is used for determination, as described above,
then it becomes important to consider the content of information to be provided and of
warnings issued in order to account for the risk that congestion will not occur. Taking into
consideration the results from analysis of the lane change possibility rate, State 2 and State 3
differ in the ease of changing lanes. Consequently, it should be appropriate to take measures
suited to traffic conditions by changing the content of services provided or by some other suchmeans. It is conceivable, for example, that the content of information provided (the message)
would be changed, or that information would be provided to a different traffic lane.
Figure 11 presents an overview of the algorithm for determining the timing of information
provision using traffic counter data. This is the algorithm created here in light of the above
investigations.

8Is traffic volume in PL greater thantraffic volume in CL2?

Q-V Graph (Tomei Expressway outward bound side,
21.52 km post: date of congestion)
125

(1)

100

Systemparameter
Q MIN

75

State 3 window frame
Cruising 1: Q (100, 160), V (55, 85)

50

Cruising 2: Q (150, 210), V (70, 100)
Passing: Q (180, 240), V (70, 100)25

Cruising 1: Q (70, 130), V (70, 100)

Yes
Determine whether total traffic volume isat or above a certain value

(2)

Yes
Check upper and lower limits of speedin every lane (speed is within window
frame)(3)

Yes
Check upper and lower limits of traffic
volume in CL2 and PL (traffic volume iswithin window frame)

Systemparameter

State 2 window frame

n
i
o
g
ra
v
)A
/h
m
(k
d
e
p
ts

No

No

No

Cruising 2: Q (140, 200), V (85, 115)
Passing: Q (180, 250), V (85, 115)

0
0

50

100

150

200

Traffic volume (PCU/5 minutes)

250

Systemparameter

No

Window frame determination(4)
Systemparameter

Roadside system

Yes

Traffic flow
detection
SensorsYes
Is the lane change ratio for changing from CL2 to
CL1 equal to or greater than a certain value?

Message
sign

DSRC
radio

Traffic lane utilization rate adjusting

DSRC
radio
Sag & tunnel
entrance sections

Initiate information
provision for traffic lane
utilization rate
regularization

provision for traffic lane
utilization rate
regularization

Vehicle with on-board unit

Figure11. Overview of traffic lane utilization rate regularization system and the algorithm used by that system

Conditions for information provision(1) The traffic volume in PL is greater than that in CL2, and the total traffic volume in every
lane meets or exceeds a threshold value.
(2) The upper and lower limits of traffic speed in every lane are checked and the speed isfound to be within the frame (State 2 or State 3).
(3) The upper and lower limits of traffic volume in every lane are checked and the traffic
volume is found to be within the frame (State 2 or State 3).
(4) The lane change possibility rate for a change from CL2 to CL1 is at or above a certain
fixed level.
A situation in which all of these conditions are met is determined to define the timing forprovision of the traffic lane utilization rate adjusting service. In the event that thedetermination of (4) is to be made from existing traffic counter data, however, then alternative
methods will be required, such as the use of spot speeds for individual vehicles rather thanaggregated data.
6. Conclusion
The analysis of actual data regarding times of congestion in sag sections confirmed that,
immediately before congestion occurs, traffic concentrates in the passing lane so that thetraffic lane utilization become imbalanced, and that congestion begins to occur first in the9passing lane, where traffic density is higher. We found the fact that traffic capacity in the 1st
cruising lane had available leeway even when the congestion begins to occur, and a traffic
lane utilization rate adjusting service was considered as a countermeasure for congestion insag sections. This yielded the following conclusions:
・Evaluation that used the lane change possibility rate as an indicator revealed it possible toimplement a traffic lane utilization rate adjusting service by means of information provision.
・Algorithms that determine the timing for provision of information based on traffic counter
data were studied.
7. Future consideration
The authers are going to study effective services by conducting continuous measurements andanalysis of traffic behavior as it relates the formation of groups of vehicles, the reduction of
speed farther backward from the beginning point where the shock wave is generated, andother such factors. Verification will also be conducted by means of testing in a virtual testing
environment(4) that combines traffic simulator and driving simulator, as well as in field
operational tests. The authers plan to move ahead with research aimed at the practicalapplication of services in order to bring about a full-fledged ITS society in the year 2007, as
proposed by the Smartway Project Committee.
8. References(1) Takashi OGUCHI, Masahiko KATAKURA, Masanori SHIKADA**. "Case Study ofCongestion Characteristics at Bottlenecks Formed by Expressway Road Sections with
Uninterrupted Traffic Flow," in Expressways and Automobiles, Vol. 44, No. 12 (2001) (inJapanese).
(2) Yoshitsugu YAMADA**, Shigeo ABE**, Hiroki NAGASE**. "Congestion
Countermeasures Utilizing LED Information Boards," Proceedings of the 23rd
Conference of the Japan Society of Traffic Engineers (October 2004) (in Japanese).
(3) Takashi OGUCHI. "Traffic Congestion Countermeasures on Expressways," Traffic
Engineering, Vol. 33, No. 6 (1998) (in Japanese).
(4) M. Kuwahara, S. Tanaka, M. Kano, M. Furukawa, K. Honda, K. Maruoka, T. Yamamoto,T. Shiraishi, H. Hanabusa, and N. Webster. "An Enhanced Traffic Simulation System For
Interactive Traffic Environment," 2005 IEEE Intelligent Vehicles Symposium (June2005).

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