16th Road Safety on Four Continents Conference
Beijing, China 15-17 May 2013
STUDY ON THE SAFETY LENGTH OF ACCELERATION AND
DECELERATION LANE OF LEFT-SIDE RAMP ON FREEWAY
ZHOU Jin
Highway Bureau of Transport Department of Jiangsu Province
No.69 Shigu Road Nanjing Jiangsu China
E-mail: [email protected]
FANG Jing
Research Institute of Highway Ministry of Transport
No.8 XiTuCheng Road Beijing China
E-mail: [email protected]
ZHOU Rong-gui
Research Institute of Highway Ministry of Transport
No.8 XiTuCheng Road Beijing China
E-mail: [email protected]
ABSTRACT
Acceleration and deceleration lanes are the critical parts which ensure safety maneuver between
main lane and ramp. Due to terrain constraints, the left exit or left entry ramp used in the
interchange design doesn’t accord with the right-hand driving habits and drivers’ expectations.
To ensure the safety and efficiency of the operation, it’s necessary to study the safety length of
the acceleration and deceleration lanes on left-side ramp considering the vehicles’ operating
characteristic of merging or diverging. According to the headway distribution of main lane in
merging area, the probability model of vehicles in the acceleration lane merged into the main
lane was established based on the acceptance gap theory. The length of the acceleration lane
can be calculated under the different operating speed and merging probabilities. Based on
Secondary reduction theory, the required safety length of deceleration lane driving from the
inner lane to the left-side exit ramp was determined to ensure that those diverging vehicles do
not affect the operation of the vehicles on the main lanes. The recommended length of
acceleration and deceleration lane will provide a technical sustain to the safety design of
interchange.
16th Road Safety on Four Continents Conference
Beijing, China 15-17 May 2013
1 INTRODUCTION
Due to the influence of terrain conditions, urban planning, regional economic development and
road network, more and more left-side ramps are used in the design of interchange in recent
time. Considering the inside lane is the fast lane with higher speed and the outside lane is the
one with lower speed because of the right-hand driving habits and speed management, the
right-side entrance and exit ramp is widely used and regarded as the best type during the
interchange design. In contrast, more concentrations should be given on the design of left-side
ramp about the operating safety and capacity which does not match the driving expectations.
And the safety level of the left-side ramp is generally not high during the current operations.
Foreign scholars have carried out some useful research on the left-side ramp. The design
method is given in the Green Book and MUTCD for the left-side ramp. Because there is rare
focus on the left-side ramp, there are no details about the left-side ramp design in the Revision
of China’s design specification for highway alignment (JTG D20-2006). Along with more and
more using of the left-side ramp, it is necessary to carry out some researches on the left-side
ramp and propose critical design specifications and develop security measures to improve its
operational safety.
2 FIELD STUDY
In order to determine the safety length of acceleration and deceleration lane of left-side ramp,
the vehicle operating feature were observed on left-side ramp. Five interchanges with typical
left-side ramps were selected in Qingdao-Yinchuan freeway. Field study includes three aspects,
first is the vehicle operating speed distribution of interchange mainline and ramp; second is the
traffic operating characteristics of merging and diverging zone; third is the driving behavior and
psychophysiological response on the left-side ramp.
Vehicles speed datum is collected by radar guns and vehicle’s acceleration and
deceleration speed are measured using GPS. Vehicles running path are surveyed with camera
and diver’s electrocardiogram are recorded by Holter.
3 DATA ANALYSIS
3.1 Operating speed of left-side exit ramp
The operating speed datum from the 2km exit sign to the end of deceleration lane are
collected and shown in figure1.
16th Road Safety on Four Continents Conference
Beijing, China 15-17 May 2013
160
140
Speed (km/h)
120
100
（
）
80
60
40
20
Deceleration
Lane
Tapered Section
500m Exit Sign
1km Exit Sign
2km Exit Sign
0
Figure 1. Operating speed of left-side exit ramp
As shown in the figure, the operating speed characteristics include:
1) From 2km exit sign to 1km exit sign, the vehicle operating speed are 95~135 km/h
which is consistent with the mainline speed.
2) From 1km exit sign to 500m exit sign, the vehicle operating speed have a decelerating
trend and reduced 5~10km/h.
3) After 500m exit sign, the vehicle begins to decelerate and operating speed dropped to
70~100km/h at the beginning of tapered section of deceleration lane.
4) When driving on the deceleration lane, the operating speed of the vehicles does not
show a general tendency. Some of the vehicles have been in the deceleration state, while some
of the vehicles have been in the first acceleration and then deceleration. But at the entrance of
ramp the operating speed drops below 80km/h.
It can be found by operating speed variation characteristics that the vehicles have begun
slowing down before they drive into the deceleration lane. And the deceleration of the
vehicles in the inside lane will influence the forward vehicles in mainline.
3.2 Operating speed of left-side entrance ramp
The operating speed datum from the start of ramp to the end of acceleration lane are collected
and shown in figure2.
16th Road Safety on Four Continents Conference
Beijing, China 15-17 May 2013
Ramp
Acceleration
Lane
Tapered
Section
Figure 2. Operating speed of left-side entrance ramp
As shown in the figure 2, the operating speed characteristics include:
1) At the entrance ramp, the operating speed maintains the acceleration, which is relatively
stable.
2) Into the acceleration lane, the operating speed become significantly higher and increase
to 100km/h when changing lane into the mainline.
It can be found by operating speed variation characteristics that the operating speed of
entrance mainline is lower than the design speed though the operating speed is increased in
the acceleration lane. Therefore, the length of the acceleration lane must be increased to meet
the high-speed merging demand for safe operation.
3.3 Driving psychophysiological response of left-side entrance ramp
The growth ratio of heart rate is defined as the indicators to measure the driver's physiological
and psychological change which reflects the tension of the driver.
The operating speed datum and the growth ratio of heart rate in left-side ramp are collected
and shown in figure3.
operating speed
growth ratio of heart rate
90
80
70
End of ramp
Middle of ramp
Exit ramp
20
Diverging nose
60
50
40
30
Deceleration lane
Growth ratio of heart rate(%)
Speed(km/h)
16th Road Safety on Four Continents Conference
Beijing, China 15-17 May 2013
Figure 3. Operating speed and growth ration of heart rate of left-side exit ramp
It can be concluded from the figure3 that the growth ratio of heart rate does not change
much. But it is greater than the critical threshold value of 30% indicating the driving is
uncomfortable and less stable. Due to the vehicle deceleration inadequate, therefore, it is
recommended to increase the length of the deceleration lane to reduce driver tension.
3.4 Driving psychophysiological response of left-side entrance ramp
The operating speed datum and the growth ratio heart rate in left entrance ramp are collected
and shown in figure4.
It can be found from the figure4, although the growth ratio of heart rate does not change
much. It will increase when vehicles drive into the acceleration lane indicating that the work
loads of drivers are increasing. Because the driver must increase the speed as possible when
changing lane into the mainline, it is recommended to increase the length of the acceleration
lane to reduce driver tension.
Merging nose
Entrance ramp
Middle of ramp
90
80
70
60
50
40
30
20
Acceleration lane
operating speed
growth ratio of heart rate
Start of ramp
Growth ratio of heart rate(%)
Speed(km/h)
16th Road Safety on Four Continents Conference
Beijing, China 15-17 May 2013
Figure 4. Operating speed and growth ration of heart rate of left-side entrance ramp
4 LENGTH OF ACCELERATION LANE OF LEFT-SIDE RAMP
As shown in Figure5, the acceleration lane should include three parts, the acceleration section
(L1), the merging section (L2) and the tapered section (L3).
Figure 5. Acceleration lane of left-side ramp
4.1 Length of tapered section (L3)
The tapered section is defined as the one which the vehicles traverse from the acceleration
lane to the mainline carriageway. It is generally calculated by the time the vehicles traverse
one lane. The traverse time is taken 3.5s and traverse speed with the speed difference is not
higher than 20km/h as a control standard. the length of the tapered section are calculated and
shown in Table1:
16th Road Safety on Four Continents Conference
Beijing, China 15-17 May 2013
Table 1. The length of tapered section of acceleration lane
Design speed
(km/h)
120
100
80
Traverse speed
(km/h)
100
90
70
Calculated
（ m）
83
75
58
Recommended
(m)
90
75
60
4.2 The length of acceleration section and the merging section (L1+L2)
Probality(%)
The numbers of gaps of merging vehicles are decided by the traffic volume of the inside lane
of mainline. The greater the traffic volume is, the fewer the numbers of gaps are. Thus, in
order to prevent vehicle platoon on the acceleration lane, relatively longer length of
acceleration lane are required.
The distances are decided by the traffic volume of mainline. The length of the acceleration
lane should be determined with the level of service and can be calculated by a probabilistic
model.
Based on the merging probability model, the acceleration lane distance probability
distribution model under different traffic volume can be established.
110
100
90
80
70
60
50
40
30
20
10
0
0
50
100
150
200
250
300
350
400
450
500
550
600
Length of acceleration lane（m）
Figure 6. Relations between the length of acceleration lane and merging probability in
mainline volume 2600
When the mainline speed is 120km/h and the ramp speed is 60km/h, the volume of
mainline is 2600veh/h and ramp volume is 600veh/h, 100% and 15% of the ramp vehicles’
traveling distance are more than 270 meters and over 390 meters respectively, and 5% of the
ramp vehicles’ traveling distance of over 460 meters. So the length of the acceleration lane
should be 390 meters to make 85% of the merging the mainline.
16th Road Safety on Four Continents Conference
Beijing, China 15-17 May 2013
Based on the application of the merging probability model and the average driving distance
model, the length of the acceleration lane under different of design speed and mainline traffic
volume can be calculated and shown in Table 2:
Table 2. The length of acceleration lane of left-side ramp
Design speed(km/h)
120
100
80
Acceleration lane length(m)
350
230
200
5 LENGTH OF DECELERATION LANE OF LEFT-SIDE RAMP
As shown in Figure7, the deceleration lane should include two parts, the tapered section (L1)
and the deceleration section (L2).
Figure 7. Deceleration lane of left-side ramp
5.1 The length of tapered section (L1)
The calculation of the length of tapered section of deceleration lane is similar to the one of
acceleration lane. Based on the observed results, diverging speed is 75% of the design speed
of mainline. The traverse time still take 3.5s and the length of the tapered section of
deceleration lane are calculated and shown in Table 3:
Table 3. The length of tapered section of deceleration lane
Design speed
(km/h)
120
100
80
Traverse speed
(km/h)
90
75
64
Calculated
（ m）
87
73
62
Recommended
(m)
90
70
60
16th Road Safety on Four Continents Conference
Beijing, China 15-17 May 2013
5.2 The length of deceleration section (L2)
According to the deceleration of the vehicle and driving behavior, the deceleration of the
vehicle can be divided into two stages.
It’s the first deceleration stage by the driver to release acceleration pedal while vehicle
driving into deceleration lane. The deceleration rate is 1.0~1.5m/s2. According to the
differential of the mainline and ramp design speed, the length of the first stage can be
calculated. The results are shown in Table 4.
Table 4. The length of first stage of deceleration
Mainline Design
speed(km/h)
120
100
80
80
90
60
90
80
Ramp design speed(km/h)
50
40
90
90
80
80
65
65
35
90
80
65
30
90
80
65
If the driver found that he didn’t achieve the desired deceleration effect after first
deceleration stage, he will begin using the brake pedal for the second deceleration stage. After
two deceleration stages, the vehicle speed is reduced to the design speed of the ramp.
The deceleration rate is 1.5~3.5m/s2 at the second stage. According to the speed in the end
of the first stage and ramp design speed, the length of the second stage can be calculated and
shown in Table 5.
Table 5. The length of second stage of deceleration
Mainline Design
speed(km/h)
120
100
80
80
45
60
90
55
Ramp design speed(km/h)
50
40
35
100
120
125
70
85
90
30
45
50
30
130
95
55
5.3 The length of deceleration lane
According to the first and second deceleration stage length, the length of deceleration lane are
calculated and shown in Table 6:
Table6. The length of deceleration lane of left-side ramp
Design speed(km/h)
120
100
80
Acceleration lane length(m)
175
145
115
16th Road Safety on Four Continents Conference
Beijing, China 15-17 May 2013
6 CONCLUSION
The lengths of the acceleration and deceleration lane of left-side ramp were determined by
observing the operating characteristics, operating speed variation and the driving behavior of
the left-side ramp.
On the Left-side entrance ramp, vehicles from the ramp to the mainline need to accelerate
to a certain speed for safety merging. On the left-side exit ramp, in order to ensure the safety
of diverging area, the deceleration vehicle can not affect the safety of the upstream straight
vehicles. Thus, the left-Side ramp should have not only the acceleration and deceleration lane,
but also the auxiliary lane for the safety.
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16th Road Safety on Four Continents Conference
Beijing, China 15-17 May 2013
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