Transportation-Related Human Factors in High-Altitude Regions: Review, Needs, and Novelties

Oxygen and Temperature Levels

The high altitude causes the barometric pressure to decrease, spreading further apart the oxygen molecules [8]. Thus, the oxygen content in each breath would be less, resulting in reduced blood oxygen levels and inefficient oxygen delivery to the brain, tissues, and mussels. At zero altitude, the oxygen level is about 21%. For every 300 m above, the oxygen level is reduced by 1%. For example, at an altitude of 900 m, the oxygen level is 18%. Similarly, at a given altitude, every 1% oxygen above 21% will reduce the given altitude (to an equivalent altitude) by 300 m. For example, at an altitude of 5000 m, people breathing 27% oxygen will have an equivalent altitude of 3200 m (1800 m reduction for a 6% oxygen level increase). This equivalent altitude is tolerable for people with some altitude acclimatization. Oxygen enrichment of room air at high altitudes improves sleep quality, daily working efficiency, and neuropsychological function. In addition, temperature decreases at high altitudes. In the absence of snow (or rain) and clouds, the temperature reduces by about 1°Celsius per 100 m up in altitude. Otherwise, the drop in temperature is 0.6°C per 100 m [9].

Perception-Reaction Time

The perception-reaction time (PRT) is commonly defined as the time it takes for a driver to perceive and respond to hazardous situations [10, 11]. The PRT comprises the following four elements, see Transportation Association of Canada [12]: (a) detection using vision capabilities to see a visual signal (detection), (b) identifying the signal and thus understanding the stimulus (Identification), (c) deciding what action to take in response to the stimulus, such as applying the brakes and turning the steering wheel (decision), and initiating the action decided upon (response). The PRT is significant in road design as it can provide adequate sight distance for drivers to avoid potential hazards.

Visual Search Mode

The drivers’ hazard perception is a part of the handling process of hazard information [13], which can be divided into three stages: visual cognition, judgment, decisions, and driving operation. As visual cognition, an excellent visual search mode is an essential prerequisite for drivers to timely and accurately perceive dangerous information while driving. Such data can ensure that drivers respond quickly and appropriately to unexpected hazardous conditions.

Hazard Perception Ability

The hazard perception skill (HPS) refers to the driver’s ability to detect and respond to potential hazards [14]. Many studies have discovered that driving experiences, road environments, and hazard scenarios may affect drivers’ HPS. For example, as the altitude increases, the air is thinner, and the driver’s psychological state will change to varying degrees, such as loss of vision, change of dynamic vision, and decline of reaction ability [15]. Drivers are unaware of their physical or mental changes at high altitudes, impairing their perception of suddenly occurring traffic hazards.

Driving Style

Depending on various personal characteristics and driving habits, drivers have different driving styles, significantly affecting their operation and reaction when faced with a potential hazard. Recently, the graphical modeling method has been confirmed to help better understand the individual features of driving behaviors. For instance, Chen F, et al. [7] developed a driving habit graph (DHG) to model drivers’ overall driving style. The DHG, composed of macro- nodes and arcs, provided a comprehensive understanding of the specific driving style of a driver. Wang C, et al. [16] applied summative statistics and graph construction to model aggressive driving behaviors and found that the graph construction achieved better performance over statistics models.

Vehicle Operating Speed

Vehicle operating speed reflects the significant influence of driver characteristics, road geometry and environment, and vehicle performance on the actual driving. The low- oxygen environment in plateau areas negatively impacts driving performance and vehicle mechanical performance. The vehicle acceleration time and distance extend while the maximum speed and grade ability decrease. As the oxygen content decreases for the driver, various cognitive abilities, including vision, hearing, memory, thinking, and attention, will decrease. The influence of the plateau environment on operating speed has been evaluated, and operating speed models for the plateau region in China have been developed [16]. For trucks, Lei et al. [17] found that altitude substantially affects the test truck’s decelerating and accelerating performance. The truck’s speed decreased faster on steep grades and increased slower on gentle grades as the altitude increased.

Walking Speed

The walking speed of pedestrians in high-altitude areas well reflects the influence of the plateau environment on pedestrians’ participation in traffic activities. As the altitude increases, the oxygen saturation of pedestrians decreases and the heart rate increases. In addition, with the decrease in the oxygen content, the physiological state of pedestrians will gradually fatigue, and the exercise capacity will decrease to a certain extent, reflecting the reduction in pedestrian walking speed at high altitudes. The influence of altitude on the walking speed of pedestrians in tunnel evacuation was evaluated by Yan G, et al. [18]. The indicators included blood oxygen saturation, heart rate, and subjective fatigue degree. The corresponding subtraction coefficient was introduced to reflect the influence of various factors on walking speed. The results show that the low oxygen environment in the plateau area negatively impacts pedestrians. For males aged 20-30, the average walking speeds inside the tunnel were 1.2 m/s, 1.0 m/s, and 0.8 m/s for altitudes of 500 m, 1950 m, and 3850 m, respectively.

Drivers of Vehicles

High altitude can significantly increase a driver’s mental workload and reduce situation awareness, see Wang X, et al. [19]. Mental workload is the portion of information processing capacity required to perform a driving task. A workload that is too high or too low is not conducive to driving safety. In addition, the increase in mental workload can lead to a rise in aggressive traffic violations. The high altitude affects driver features, such as perception-reaction time, hazard perception ability, visual search mode, and driving style. Therefore, improving situational awareness and reducing mental workload can effectively mitigate the driving risk from the high-altitude environment.

Pedestrians

People travelling at high altitudes tend to control their speed more intentionally and will try to stroll to adapt to the high-altitude environment. The walking pace is coordinated with breathing to find a suitable walking rhythm. This reflects that the overall pedestrian traffic in high-altitude areas is relatively slow and maintains an even rhythm. Intuitively, the high-altitude environment has adversely impacted traffic activities, and pedestrians are likelier to feel tired and weak [18].

Cyclists

More people are beginning to do cycling at high altitudes as a challenging, adventurous, and exploratory trip. In addition, many professional cyclists are trained at high altitudes. The thinner air at high altitudes makes the air resistant, making cycling training more effective. Riding at moderate altitudes for the first time may improve riding performance on lower altitude terrains, as the benefit of reducing air resistance outweighs the reduction in maximal aerobic power. However, Hahn AG, et al. [20] showed that long-term cycling at high altitudes led to adverse effects, such as fatigue and altitude sickness. Garvican L, et al. [21] pointed out after observing the three-week high-altitude training of cyclists that high- altitude training did not significantly increase the maximum value of hemoglobin but only increased its concentration. However, it could improve the athlete’s level.

Pedestrians-Cyclists

Pedestrians and bicycles in high-altitude areas also face serious problems, including the unpredictable impact of climatic phenomena such as heavy fog and wind in high- altitude areas on road users. A critical reason fog affects traffic safety is that it reduces the visibility of the road, which seriously interferes with pedestrians and cyclists obtaining road condition information and road environment information. This, in turn, affects decision-making behavior and the occurrence of traffic accidents. The impact of strong winds on traffic safety is mainly reflected in the following points. First, strong winds will increase vehicle driving resistance and reduce vehicle stability. Second, strong winds will cause a great disturbance to pedestrians or people riding non-motor vehicles. For example, blowing dust into their eyes or disturbing their clothes and hats will seriously distract them. The strong wind affects the walking or cycling routes, significantly increasing the probability of colliding with motor vehicles, thus causing traffic accidents [22]. Third, the low temperature in high-altitude areas cracks the road surface, causing the road surface to be discontinuous, providing a channel for precipitation to infiltrate. When the temperature turns warm, the road surface becomes muddy and slippery, reducing the adhesion of the road to the tires, and making it easier for cyclists and pedestrians to skid. Finally, in some high-altitude areas with poor air quality, cyclists and pedestrians face more significant impacts from inhalable particulate matter and smog than cars in confined spaces because cyclists and pedestrians are exposed to the road environment [23]. The world’s longest urban transit pedestrian system has been developed in Lhasa, China. The system is divided into four layers of ring roads and has formed a high-density bicycle lane system with the highest altitude in the world. More than 60% of the roads in the main urban area have been equipped with bicycle lanes [24].

Some high-altitude areas involve spiritual and cultural pursuits of holy places, attracting many pilgrims and considerably slowing traffic. There are two types of walkers: single walkers and multi-person walkers. Many Tibetans, taking pilgrims in the plateau areas, kowtow to the ground for the “body” respect to show their belief in Buddhism (Figure 2). At the same time, they constantly recite incantations in their mouths and travel thousands of kilometers for pilgrimage to the Jokhang Temple in Lhasa, a UNESCO’s World Heritage, showing great piety and persistence. Notably, the speed of the pilgrims is much less than that of normal walkers.

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A: Pilgrims

B: Jokhang Temple Figure 2: Pedestrians’ pilgrims in the plateau areas.

Passengers

Passengers who travel to high-altitude regions would be exposed to hypoxia before and after riding the trains or public transportation. Therefore, passengers should travel first to a low altitude in the area to get used to the environment, then take public transit to a higher altitude; see Tibet Train Travel [25]. For example, tourists to Tibet can first travel to Lhasa (3650 m), the capital of Tibet, then take the train to Xining

(5072 m), the capital of Qinghai. On the other hand, starting the trip from Qinghai may cause some unease on the way to Lhasa by train, thus affecting their tour program. In addition, at an altitude of more than 4500 m, some weird things may happen, such as ink-pen leaks, the explosion of vacuum- sealed food packages, and the failure of some laptops and digital music players. Fig. 3 shows a staff serving passengers on the bullet train from Lhasa to Nyingchi [26].

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Train passengers at high altitudes are advised to be cautious regarding several aspects [25]. First, they should take light, high-carbohydrate meals for more energy and avoid alcohol since it may increase the dehydration risk. Second, trains may stop at some high-altitude stations with beautiful views, and they should not run at the height to avoid feeling uncomfortable afterwards due to lack of oxygen. Third, since the high altitude allows the sun’s solar radiation to strike the earth more intensely, passengers can use sunscreen, sunglasses, and lip creams to protect their eyes and skin. Finally, tourists with heart problems or high/ low blood pressure should check with doctors before visiting high altitudes.

Vehicle Performance

Does high altitude affect a vehicle’s performance? The answer is Yes. High altitude significantly affects vehicle performance, causing the vehicle to lose power [27]. This happens because less air is fed to the internal combustion engine. Generally, an engine loses 3% of its rated power for every about 300 m of altitude gain. That is, for an altitude of 2000 m, the engine is expected to lose 20% of your horsepower. However, the amount of lost horsepower depends on the engine size and configuration. In addition to power loss, high altitude can affect vehicle performance in two ways. First, a low-octane level should be required at high altitudes since high-octane fuels have no advantage in deficient-oxygen environments. Second, the lower temperatures and air pressure at high altitudes cause the tire pressure to drop, causing reduced handling and breaking and a drop in fuel mileage. It should be noted, however, that modern vehicles include solid air and fuel systems to compensate for air density changes.

Transportation Design

Transportation users have many parameters that affect the design and operation of transportation facilities. Table 1 summarizes, for each transportation user, the human-factor parameters and related design and operational elements. As noted, the parameters related to drivers, pedestrians, and cyclists (e.g., PRT and driver deceleration rate) mainly affect highway geometric design. In contrast, the passengers’ parameter (low oxygen) affects train and public transit design. The main design elements include stopping sight distance (SD), passing SD, decision SD, crossing SD at an intersection controlled by a stop sign on the minor road [12], railway crossing SD [28], traffic control devices [29], roundabout design [30], freeway acceleration and deceleration lanes, and sag vertical curve design [31].

For example, the stopping sight distance (SSD) is a function of the perception reaction time and deceleration rate. It is the sum of the distance travelled during the perception-reaction time and the braking distance. The braking distance is the distance to stop a vehicle once the brakes have been applied. For a level roadway, SSD is given by [12, 31] 2 0.278 0.039 v SSD Vt a = + (1) Where SSD = stopping sight distance (m), V = design speed (km/h), t = perception-reaction time (s), and a = deceleration rate (m/s2). The SSD for plateau regions is expected to be larger than for plain regions because the PRT is longer and the deceleration rate is lower.

Traffic Operations

Human factors also affect several highway operational elements, as shown in Table 1. These include yellow interval determination to avoid the dilemma zone [32], the minimum green interval for traffic signals [33], and pedestrian crossing SD [34]. For example, the minimum green interval for traffic signals on level approach is given by

0.278 ( ) 2 0.278 V W L I t a V + = + + (2) Where I = intergreen (yellow plus all-red) interval (s), t = driver’s PRT at the onset of the yellow interval(s), V = approach speed (km/h), a = deceleration rate (m/s2), W =

intersection width (m), and L = vehicle length (m). As noted, this formula depends on the driver’s PRT and deceleration rate. Therefore, larger values for I would be required in plateau regions.

Train Operation Technology

Airplanes and individual rooms have confined environments. Therefore, oxygen enrichment of the air is straightforward. However, for trains, oxygen enrichment is more challenging for several reasons [35]: (a) the volume of air oxygenated in a train (cars and gangways) is much larger, (b) oxygen concentration inside the train must be maintained despite the opening of the door at stations, and (3) waste must be disposed of without affecting the oxygen- enriched environment. Therefore, this train operation technology represents an extraordinary innovation in high- altitude transportation.

For example, the train from Golmud, Qinghai (2809 m) to Lhasa, Tibet (3658 m) in China travels at an average altitude of 4500 m for more than 14 hours [36]. Because of the high altitude, passengers may be exposed to sustained hypoxia. Therefore, the oxygen concentration in the train (which has 16 passenger cars) has been increased to 25% using oxygen generators in the train cars to address this problem. The generator typically produces 40% to 50% oxygen, which is added to the ventilation air of each car to achieve 24% to 25% oxygen concentration in the car. An oxygen concentration of 25% reduces the equivalent altitude by 1200 m. Therefore, at the train’s highest altitude (5000 m), the cabin altitude is reduced to 3800 m, which is greater than the altitude of the Lhasa terminal station. Each car’s oxygen generator unit has a panel that passengers can view, indicating the oxygen level. In addition, the train’s temperature is adjusted with the altitude, and the windows have UV-glazed coating for protection from high-altitude rays.

Railway Construction

Railway construction at high altitudes faces three main challenges: lack of oxygen, fragile ecosystem, and permafrost (Figure 4). An example is the bullet train’s electrified railway in southwest China’s Tibet. Autonomous Region (Lhasa to Nyingchi) with a design speed of 160 km/h. The single-line railway has a length of 435 km, involving 47 tunnels and 121 bridges, accounting for 75% of the route and posing tremendous challenges for construction workers. More than 130,000 workers were involved in building the railway, most of which had an altitude of more than 3,000 m.

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C: Tunnel

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D: Bridge Figure 4: Tunnel and a bridge along the bullet train route from Lhasa to Nyingchi.

Proper aid for the construction workers was provided during the Qinghai–Tibet railway track construction, involving 80,000 workers, which was formidable. The project had 17 oxygen generators and 25 clinics with hyperbaric chambers along the track during the railway construction. In addition, the designers carefully selected the route to avoid destroying vegetation and the natural habitat of wild animals. Further, they ensured stability in permafrost regions by installing gravel embankments [35].

Engineering challenges at two tunnels are described next to demonstrate the remarkable construction innovations in this project [36]. First, the excavations were done using hand drills for the Fenghuoshan Tunnel (4905 m), which is the world’s highest-altitude railroad tunnel. The hypoxia was partially relieved using large volumes of high-concentration oxygen produced from generators outside the tunnel. First, the generators absorb nitrogen when the air is pumped through them, creating a high oxygen concentration. Then, the oxygen was pumped to the end of the tunnel in a pipe and released. The increase in oxygen was equivalent to an altitude reduction of 1200 m. In addition, a small room was constructed where workers could inhale oxygen. Second, acute mountain sickness among workers occurred during construction at the Mount Kunlun tunnel (4660 m), which is 1686 m long and believed to be the world’s longest permafrost tunnel. In this project, the medical examination of nearly 75,000 over 2.5 years, the incidence of acute mountain sickness ranged from 45% to 95%, depending on the altitude.

Well-Connected Transportation Network

China has increasingly invested in constructing Tibet’s transportation infrastructure over the past 60 years to promote economic and social development [37], as shown in Figure 5.

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The total road mileage grew from 7300 km in 1959 to 89,343 in 2017 (more than ten times increase). The developed road and train network is centred in Lhasa. Tibet has completed a five-year plan (2016-2020) for comprehensive transportation development. In addition, the region has emphasized road development in the countryside by building and upgrading 4,500 km of rural roads to link its 33 townships and 533 villages to promote poverty reduction further. Rural passenger transport lines are also launched to solve the transport difficulties for local farmers and herders.

Visual Perception

Zhang D, et al. [40] analyzed the eye movement behaviors of drivers when faced with traffic hazards in the plateau environment to examine the impact level of altitude on drivers’ perception and reaction. Nine typical traffic hazard scenarios were carried out at four locations with four different altitudes, including Linzhi (2995 m), Lhasa (3650 m), Naqu (4460 m), and Yanghu Scenic Area (4998 m) based on UC-WIN/ROAD driving simulation software. Then, drivers’ visual search modes were analyzed according to drivers’ eye movement data collected by ASL Mobile Eye monocular eye tracker. Then, the first fixation time of hazard scenarios, percentage of fixation duration in the area of interest (PFD), and average saccade amplitude (ASA) were analyzed. For example, the PFD results for different driving experiences are shown in Fig. 6. Then, the drivers at higher altitudes perceived the situation in hazard scenarios later with larger PFD in the hazardous areas and had a smaller ASA than those in low-altitude areas. These results indicated the effects of a low-oxygen environment on visual sensitivity, perception and processing ability, and visual search mode. Moreover, experienced drivers had smaller PFD and larger ASA, while drivers with a short acclimation period had smaller PFD.

Hazard Perception Ability

Wang C, et al. [41] analyzed the perception ability of four hazardous experiments at Nanjing (50 m) using driving simulation software. The hazard perception- reaction time could be specified. Significant weak negative correlations were found between perception-reaction time and age, acclimation period, and driving experience (-0.106, -0.175, and -0.191, respectively). There were no significant relationships between perception-reaction time and gender, and scenario. Then, the K-means clustering divided the drivers into three groups (Good, Medium, and Poor). Finally, the three groups’ marginal effects of linear regression models were calculated to augment the comparison. As expected, the elevation positively correlated with the perception-reaction time but indicated variance of influence effects in the three groups.

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Hazard Perception-Response Mode

After analyzing the perception ability, Wang C, et al. [42] proposed a graph construction approach to model drivers’ hazard response mode hazards in plateau areas. The simulation experiments were successively conducted in the same five cities used for the hazard perception ability study. Then, according to the graph construction approach, 4 hazard response modes (HRM) for drivers were extracted (Figure 7). The symbols in the figure are defined as follows: SA (steering wheel degree), TH (throttling degree), BR (braking degree), AX (lateral acceleration), AY (longitudinal acceleration), VX (lateral speed), and VY (longitudinal speed).

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The drivers of Mode 1 tended to have good perception ability while taking efficient and expected behaviors. The drivers of Mode 3 were found to have longer hazard- response times than those of Modes 1-2. They first took the avoidance operations being turning the steering wheel. Then, they braked to pass the hazardous vehicle or pedestrian. By contrast, the drivers of Mode 4 were observed to have the worst perception ability and might be panic-stricken when encountering the hazard situations. The effects of significant variables, including scenario types, altitude, acclimation period, driving experience, and gender, filled the knowledge into the construction of HRM and risk perception ability of plateau drivers. These results showed that constructing HRM to model the driving styles of plateau drivers is feasible and effective, enabling future assistance driving systems to be better customized for drivers in such a particular condition.

Vehicle Operating Speed

To study the influence of the harsh environment in plateau areas on the operating speed of vehicles, Wang C, et al. [16] established advanced speed prediction models based on the observed actual speed data. First, the speed characteristics were observed on Lalin Highway and National Highway 318 with Bushnell’s handheld radar speedometer. Second, the stepwise regression method was proposed to determine the significant parameters and present the operating speed prediction models for the two highways. Finally, the reserved test group data was conducted to prove the validity and practicality of the proposed models. Compared to the traditional methods, the established models can produce more accurate prediction results and deeply examine the nonlinear relationships between parameters and the predicted operating speed.

Minimum Curve Radius

The peculiar natural geographical environment in high altitudes significantly influences the drivers’ psychophysiological states, enhancing drivers’ mental workload and deteriorating driving safety. Therefore, it is significant to incorporate the impacts of low-voltage and hypoxia conditions of plateau areas on drivers when examining the minimum radius of the horizontal curve applied to two-lane highways in plateau areas. Chen F, et al. [7] conducted field experiments based on the segments from Nyingchi to the Mountain Shegyla on G318. Besides, the model for the simulated experiment is established based on UC/Win-Road software to exaggerate the adequate sample size. The consistency of the field and simulated experiments is validated through the Paired Sample T-test. Sample entropy is adopted to process the collected multi- source data, including the velocity of the vehicles, heart rate, and electroencephalogram of the driver. In addition, the principal component analysis evaluates a comprehensive index (CI), which represents drivers’ psychophysiological state. Subsequently, the correlation between CI and radius is anatomized, validating that the lateral acceleration ah is the index of greatest influence on CI from the driving dynamics perspective. Values of the minimum radius of the horizontal curve of two-lane highways in the plateau area were proposed to amend the current Chinese standards. Overall, this research may play an essential guiding role in alignment design in plateaus areas and conspicuously deepen our understanding of driving safety under low-voltage and hypoxia environments theoretically and practically.