2.1.1. Accident Characteristics Analysis
- (1)
Accident time distribution
As shown in
Figure 1, accidents were more likely to occur during the day in the Qingdao Jiaozhou Bay Subsea tunnel in 2019; the distribution of traffic accidents in the tunnel exhibits a clear “bimodal” phenomenon, with a concentration of traffic accidents during the day’s peak hours.
- (2)
Accident types
Based on the historical accident records of the underwater tunnel of the urban expressway and the tunnel’s traffic operating characteristics, this study categorizes the tunnel’s accident types into seven categories: rear-end collision, scraping, collision with the roadside ditch, rollover, fire incident, engine failure and others. The types of accidents that occurred annually in the tunnel from 2018 to 2020 are tallied using the urban expressway underwater tunnel accident record table; the statistical outcomes are shown in
Figure 2.
The majority of incidents in urban expressway underwater tunnels are rear-end collisions, with rear-finished collisions accounting for more than 45 percent of total accidents from 2018 to 2020.
As a result, this paper uses the Jiaozhou Bay underwater tunnel rear-end collision, scraping, collision with the roadside ditch, and overturning accidents as the research object, along with statistics on the distribution of accident models and spatial distribution of accidents, to investigate the impact of road factors on tunnel traffic accidents and then to discover tunnel traffic accident law.
- (3)
Type of vehicle involved
Through the above analysis, the distribution of accident models of rear-end collision, scrape, collision with the roadside ditch and rollover accidents in underwater tunnels of urban expressways were tallied, and the accident models were primarily classified into five groups: commercial vehicles, cars, SUVs, vans, and buses, as shown in
Figure 3.
As illustrated in
Figure 3, the automobile is the most common model of urban expressway undersea tunnel traffic accidents, accounting for roughly 90% of total traffic accidents in the statistical year, a proportion significantly higher than other accident models. The study of the effect of road conditions on tunnel traffic accidents is critical for tunnel traffic accident legislation.
- (4)
Accident spatial distribution
Using the Jiaozhou Bay underwater tunnel accident as the starting point, statistics of urban expressway underwater tunnel rear-end collision, scraping, collision with the roadside ditch, rollover accident spatial distribution, the road mileage stake number as the basis for division, the left and right lines of the tunnel to 400 m intervals for uniform division, and statistics of the number of traffic accidents in each road unit of the left and right lines of the tunnel, the statistician deduced that the left tunnel (“left line”) connects Qingdao from south to north, while the right tunnel (“right line”) connects Qingdao from north to south.
Figure 4b demonstrates that the number of traffic accidents in the tunnel’s right lane between YK4+400 and YK4+800 is quite low, which is strongly related to the configuration of curved portions in this segment. Clearly, road conditions have an effect on traffic collisions.
In conclusion, there is a correlation between the prevalence of urban expressway underwater tunnel accidents and road slope, curve radius, distance from the bottom of the slope, and slope length.
2.1.2. Analysis of the Correlation between Influential Variables and Accident Rate
Analysis from the perspective of system safety engineering suggests that the primary goal of preventing traffic accidents is to investigate the rules governing the occurrence of accidents and to identify their primary causes. In this paper, geometric conditions (slope, slope length, curve radius, the proportion of road length from the bottom of slope) and control measures (traffic line marking, intelligent transportation facilities) are selected as the main influencing factors of underwater tunnel accidents of urban expressways.
- (1)
Influence of slope on the occurrence of accidents
Slope () is the actual slope of the road, with a positive value indicating an uphill segment and a negative value indicating a downhill segment; this determines the accident rates for each segment.
Figure 5 shows that different slopes have varying degrees of influence on tunnel traffic accidents, and that the rate of tunnel traffic accidents varies within the same slope interval. When the longitudinal slope is greater, the number of tunnel traffic accidents increases, and vice versa.
- (2)
Influence of the curve radius on accident occurrence
Curve radius (R) is the road’s turning radius. When straight sections of road are aligned, the turning radius is recorded as ∞. The traffic accident data of each section of urban expressway underwater tunnel with a curve radius less than 3000 m, between 3000 m and 10,000 m, and greater than or equal to 10,000 m, i.e., straight sections, are compiled and counted based on the curve radius in
Figure 6.
Different curve radii have varying degrees of influence on tunnel traffic accidents, as shown in
Figure 6. From a global perspective, the downhill section of the tunnel has a significantly higher traffic accident rate than the uphill section, but whether uphill or downhill, the tunnel traffic accident rate follows the same pattern of increase in straight sections.
- (3)
Influence of slope length on accident occurrence
The actual length of the road within the statistical section is represented by the slope length (D). The accident rate is used as an indicator to compile and analyze the traffic accident data for each section of the underwater tunnel with slope lengths ranging from 100 to 800 m.
Different slope lengths have varying degrees of impact on tunnel traffic accidents, as shown in
Figure 7. In general, the downhill section of the tunnel has a significantly higher accident rate than the uphill section. In the downhill segment of the tunnel, the traffic accident rate is proportional to the length of the slope; the longer the slope, the higher the tunnel’s traffic accident rate.
- (4)
Influence of the percentage of road length from the bottom of the slope on the occurrence of accidents
This work combines the road alignment characteristics of urban submerged tunnels and selects the ratio of road length from the bottom of the slope to reflect the distance from the bottom of the tunnel slope.
Figure 8 compiles and evaluates accident data from various sections of urban motorway underwater tunnels using the accident rate as an indicator.
Figure 8 shows how different road length ratios from the slope’s base have varying degrees of impact on tunnel traffic accidents. On the tunnel’s longitudinal sections, particularly the uphill sections, the accident rate of each statistical section is lower, the value is lower, and some statistical sections have a slightly higher accident rate.
- (5)
Influence of geometric two-factor combination conditions on the occurrence of accidents
① Gradient and curve radius combinations
The relationship between different road types and tunnel traffic accidents under the combination of slope and curve radius can be discovered by studying the relationship between different road types and tunnel traffic accidents: the tunnel traffic accident rate is highest on steep downhill and straight sections, and lowest on gentle slopes and straight sections. According to the tunnel traffic accident change pattern depicted in
Figure 9 and
Figure 10, whether in the tunnel uphill or tunnel downhill section, the tunnel steep slope section always has more traffic accidents than the tunnel gentle slope section, especially in the tunnel steep downhill section.
② Combination of gradient and slope length
The correlation between various road types and traffic accidents in tunnels under the influence of slope and slope length reveals that: traffic accidents are more common in road sections with higher slope and longer slope length, and the accident rate in tunnels is significantly higher in the steep downhill sections, followed by the steep uphill sections. The rate of tunnel traffic accidents rises with an increase in slope length at a given slope, as shown by the changing pattern of tunnel traffic accidents in
Figure 11 and
Figure 12.
③ Combination of slope and length from the bottom of the slope
Studying the relationship between various road types and tunnel traffic accidents under the combination of slope and slope length reveals that: the steeper the tunnel’s slope, the shorter the proportion of the road’s length from the slope’s bottom, and the more frequently accidents occur; the accident rate in the steep downhill tunnel is also significantly higher than that of other road sections; the steep uphill road section is the second-highest accident rate location. As the percentage of road length from the slope’s bottom decreases, the rate of traffic accidents in tunnels rises, according to the shifting patterns of accidents in
Figure 13 and
Figure 14 at the same slope.
- (6)
Impact of control measures on the occurrence of accidents
In conjunction with the current situation of the Jiaozhou Bay Crossing’s improvement of control measures in recent years, the optimization of tunnel traffic markings was carried out. As a result, the focus of this paper will be on comprehensive control measures based on traffic marking. The data were collected between 23 January 2021 and 25 January 2021, prior to and following the first traffic lane change marking in the Qingdao Jiaozhou Bay Subsea Tunnel in
Table 1. For the first time, the Qingdao Jiaozhou Bay Subsea Tunnel has a total of seven lane change markings, with each section designed in the form of white dashed lines and white dashed solid lines. The following is the marking implementation strategy in
Figure 15.
The data of the Qingdao Jiaozhou Bay Subsea Tunnel were chosen from February to June 2021 after the first delineation and from February to June 2019 before the first delineation. The data set includes historical accident information such as accident time, type, and location, as well as the average daily number of vehicles in the tunnel during the statistical period in
Figure 16.
According to the statistical results of the number of traffic accidents before and after lining, the occurrence of tunnel traffic accidents is inextricably linked to the geometric conditions, which confirms the previous analysis of the influence of geometric conditions on tunnel traffic accidents.
Furthermore, there is a significant difference in the length of the tunnel traffic accident statistics before and after the delineation; therefore, in order to accurately analyze the impact of the marking design on tunnel accidents, this section chooses the accident rate as an evaluation index to compare and analyze the changes in tunnel traffic accidents before and after the delineation.
Table 2 displays the results.
The analysis of the changes in accident rates in the delineated sections of the Qingdao Jiaozhou Bay Subsea Tunnel before and after the first delineation revealed that, with the exception of delineated sections No. 3 and No. 4, tunnel traffic accident rates in the rest of the delineated sections were significantly reduced, while traffic accidents in the delineated sections No. 3 and No. 4 were elevated or slightly reduced, and the changes were not significant.
This section examines the results of the traffic accident rate calculation for the lined section of the Jiaozhou Bay underwater tunnel before and after the application of lane change markings using SPSS 22 software, and the independent sample
t-test was chosen to verify the results, which are shown in
Table 3. According to the results, at the 95% confidence interval, F = 5.035 > 0.05, equal variance is assumed between the two groups of data, and the significance Sig = 0.0490 < 0.05, indicating that there is a significant difference between the two groups of data, and the application of lane change markings has a certain impact on the Jiaozhou Bay underwater tunnel’s accident rate.
Figure 17 shows that the traffic accident rate of Qingdao Jiaozhou Bay Subsea Tunnel before and after lane marking has been reduced to a certain extent after lane change markings are applied, with a reduction of 30% to 40% after lane change markings are applied, excluding the influence of special sections such as tunnel entrances and exits, diversions and merges.