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: zhouj@jsgl.cn FANG Jing
Research Institute of Highway Ministry of Transport No.8 XiTuCheng Road Beijing China
E-mail: j.fang@rioh.cn ZHOU Rong-gui
Research Institute of Highway Ministry of Transport No.8 XiTuCheng Road Beijing China
E-mail: rg.zhou@rioh.cn
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.
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.
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.
0 20 40 60 80 100 120 140 160 2 km Ex it S ign S p ee d （ (km /h ) ） 1 km Ex it S ign 5 00 m Ex it S ign T ap er ed S ect io n D ec ele ra tion La ne
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.
Ramp Tapered
Section Acceleration
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.
30 40 50 60 70 80 90 operating speed
growth ratio of heart rate
S p eed (km /h) G row th r at io o f he ar t r at e(% ) D ecel er at io n l an e D ive rg ing nos e E x it r amp M id dl e of r am p E nd of r am p 20
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:
operating speed
growth ratio of heart rate
S p eed (km /h) G row th r at io o f h ea rt r at e(% ) S ta rt o f r amp Mi d d le o f r am p M er g ing nos e A ccel er at io n l an e E n tr an ce r am p 20 30 40 50 60 70 80 90
Table 1. The length of tapered section of acceleration lane Design speed (km/h) Traverse speed (km/h) Calculated （ m） Recommended (m) 120 100 83 90 100 90 75 75 80 70 58 60
4.2 The length of acceleration section and the merging section (L1+L2)
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.
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.
0 10 20 30 40 50 60 70 80 90 100 110 0 50 100 150 200 250 300 350 400 450 500 550 600
Length of acceleration lane（m）
P ro b al i ty ( %)
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) Acceleration lane length(m)
120 350
100 230
80 200
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) Traverse speed (km/h) Calculated （ m） Recommended (m) 120 90 87 90 100 75 73 70 80 64 62 60
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)
Ramp design speed(km/h)
80 60 50 40 35 30
120 90 90 90 90 90 90
100 80 80 80 80 80
80 65 65 65 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)
Ramp design speed(km/h)
80 60 50 40 35 30
120 45 90 100 120 125 130
100 55 70 85 90 95
80 30 45 50 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) Acceleration lane length(m)
120 175
100 145
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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