The results of this study indicated that total road network density was positively related to global cognitive function. Higher road network density not only promoted walking and cycling but also promoted the use of public transportation, such as walking to a bus station or train station, thereby increasing individual physical activity [25, 26]. A survey on the potential risk factors of AD showed that, on a global scale, inadequate physical activity is one of the two most important risk factors for the development of AD . Longitudinal studies also have confirmed the importance of physical activity in reducing dementia risk . In a study of 2492 elderly Germans, regular participation in any additional physical activity was shown to reduce the incidence of dementia by 20% over the next 4.5 years . In addition, a study of 803 elderly people living in the suburbs of Japan found that compared with those who were less active, those who claimed to have at least one day of physical activity per week had a 41% lower risk of developing AD over the next 12 years . Other longitudinal studies on the elderly have shown that compared with a sedentary lifestyle, any physical activity can reduce dementia or dementia-related mortality [31–33]. Therefore, we speculate that a dense road network can slow the progression of cognitive impairment by encouraging individuals living in the area to increase their physical activity.
With the expansion of the road network, however, the flow of motor vehicles will also increase, which will increase air pollution [34, 35]. Therefore, in this study, we tested the mediating role of air pollution. Our results showed that air pollution weakened the promotion effect of total road network density on cognitive function. Although everyone may suffer the consequences of long-term exposure to air pollution, the elderly are more vulnerable. Increasing evidence has suggested that chronic ambient air pollution exposure may have neurotoxic effects in older individuals. In a multivariable-adjusted model for global cognitive function, the results demonstrated the association of black carbon with lower cognitive performance and cognitive decline in older individuals . Another study evaluated the long-term exposure to particulate matter (PM) among 399 women 68–79 years old demonstrated an association between long-term exposure to air pollution and mild cognitive impairment . Other researchers have reported that exposure to high concentrations of air pollutants will increase the risk of AD  and vascular dementia . It is precisely because of the harmful effect of air pollution on cognition that air pollution plays a suppressing role in the relationship between road network density and global cognitive function.
The majority of studies to date have reported the negative association of air pollution with global cognition or different domains of cognition, especially memory, language, and attention. A study assessed adults age 50 years and older living in areas with higher PM2.5 and found that participants had worse cognitive function, particularly for episodic memory but not for mental status . Researchers also observed the relationship between estimated annual ozone exposure and reduced performance in specific central nervous system (CNS) functions. The results showed that the adverse effect was more pertinent to ozone-induced impairment in memory, attention, and coding ability but not in visuomotor speed . Another study that examined a variety of air pollutants also showed that air pollution was inversely associated with logical memory abilities, executive functioning, and verbal learning performance . The results of this study showed that air pollution mediated the relationship between road network density and memory, recall, language, attention, and calculation but not orientation. This finding was consistent with previous research results demonstrating that air pollution may be related to one or several dimensions of cognitive function.
Different roads have different characteristics and functions. Therefore, to further study the impact of different categories of roads on cognition, this study divided total roads into three categories. Results showed that the secondary road network density and other road network density remained positively correlated with cognitive function, but the main road network density was negatively correlated with cognitive function. The main roads generally are open to large cars with the largest traffic volume, thus connecting different cities and communicating with the outside world. The secondary road connects the main areas in the city, undertakes the main traffic tasks of the city, and forms the skeleton of the urban road network. Other roads play a role in collecting and distributing traffic and also have service functions. Compared with secondary roads and other roads, the main roads have few nonmotorized vehicles, larger traffic volume, and no restrictions on large vehicles. As a result, main roads have a limited effect on promoting people's physical activities such as walking and cycling. Thus, different categories of road have different impacts on people.
Liaoning Province is located in northeast China, from 38°43' to 43°26' north latitude. Its winter is long and cold, with a heating period of up to five months each year. In addition to vehicle exhaust emission, coal-fired central heating is a significant source of air pollution in northern cities. Thus, based on the analysis of the mediating effect of air pollution throughout the year, in this study, we selected the heating period and controlled the number of chimneys of the heating companies for analysis to reduce the impact of air pollution from heating sources. For comparison, we also selected the nonheating period to eliminate air pollution from the heating source. Results showed that in the three periods, the mediating effect of air pollution in main road network density and cognitive functions, and the suppressing effect of air pollution in other road network density and cognitive functions did not change. The secondary road network density continued to have a positive relationship with cognition. Although the role of air pollution on the relationship between secondary road network density and global cognitive function during the nonheating period was not statistically significant, it revealed a suppressing effect trend. Because it is difficult to travel in cold and snowy winter, the use of bicycles and electric bicycles decreased significantly, and the use of motor vehicles increased. At the same time, the phenomenon of temperature inversion makes it difficult for pollutants to spread. All of these factors make air pollution more serious during the heating period and reduce individual physical activities, which in turn has a greater adverse effect on cognitive function. This analysis further confirmed that the role of air pollution on the relationship between road network density and cognitive function was stable.
Our study had some limitations. First, like other cross-sectional studies, the causal directions could not be examined. Second, although we adjusted many measured demographic characteristics and health-related characteristics of people, we were unable to control all covariates, and some variables that were not included, such as a sedentary lifestyle, depression, and unhealthy diet, may have affected our estimates. Third, individual exposure to air pollution in this study was estimated based on air monitoring data and may have been imperfect measures of personal exposure to ambient air pollution. Therefore, we applied inverse spatial interpolation to minimize exposure error resulting from spatial variation in air pollution concentrations, which reduced the error of using traditional closest environmental monitoring concentration to assess exposure.
Thus far, treatment for dementia and cognitive decline is not effective. With increased aging, cognitive decline and dementia have become important public health issues that seriously affect the mental health and quality of life of the elderly. Environmental characteristics that support active aging may reduce the risk of cognitive impairment and dementia. In recent years, scholars increasingly have turned their attention to researching the impact of the BE on cognitive functions [4, 43–45]. With the continuous development of urbanization, road networks have continued to expand. In China, by the end of 2019, the road network density was 52.21 km/100 km2, an increase of 4.53 km /100 km2 over 2015 . This research found higher secondary road network density and other road network density could be beneficial to cognitive function by enhancing individual physical activity, but at the same time, this density also increased air pollution, thereby further impairing cognitive function. In addition, main road network density was positively correlated with lower cognitive function scores. Thus, when expanding road network construction, attention should be paid to reducing air pollution, thus ensuring that people can enjoy the convenience of transportation while also minimizing adverse effects on health.