## 1 Introduction
Other studies investigated the impact of traffic congestion on the environmental and socio-economic development. Time spent due to congestion and the cost of traffic accidents are not the only economic losses caused by traffic [13]. Congestion can also cause people and goods to arrive late, which affects productivity and competitiveness [14]. Because of the time spent in traffic, people feel tired, which affects their ability to work and reduces their productivity. They also feel stressed and frustrated, which indirectly affects their health and social skills [15]. However, air pollution from urban traffic has one of the greatest impacts on health. Therefore, many studies have investigated how urban traffic affects the urban environment through air pollution caused by the imperfect combustion of fuels (CO and CO\({}_{2}\)), combustion-related emissions (NO and NO\({}_{2}\)), unburned or converted hydrocarbons (benzene, toluene, etc.) and condensation substances (PM2.5, PM10) emitted by vehicles [16]. Roadside concentrations of traffic-related pollutants vary significantly due to local emissions, urban meteorological patterns, pollutant release or accumulation patterns. The COVID-19 pandemic fundamentally changed the behavior of the population. The situation, enforced by the quarantine, led to a massive decrease in urban mobility and tourism, public transport was abandoned, and the population pursued safer alternatives such as walking or cycling [7; 17]. Many researchers have attempted to analyze the dynamics during the pandemic in urban areas in order to understand the impact of COVID-19 on cities [17], including the improvement in air quality due to the decrease in traffic [18; 19; 20]. In order to assess mobility change in the period of COVID-19, global data from Google or ESRI on user mobility reports were used to describe changes in mobility [21], while for analysis of the air-quality data from EPA or satellite data from NASA were used to estimate pollution levels [17; 20]. This paper investigates how redesigning a significant intersection affects traffic in the target area and in adjacent areas. The study was conducted for a hotspot intersection that was proposed to be redesigned due to a high number of accidents. The preinvestment traffic analysis showed that the main traffic direction was north-south, followed by east-south and west-north. During the redesign, an east-west underpass was proposed, but with the restrictions of left-turns, from both directions would not have been possible. Because the preferred traffic direction was using left-turns from east and west, we hypothesized that the impact of the proposed underpass would be less than assumed by the government and the design team. To test this hypothesis, we proposed to analyze the impact of this investment on transportation infrastructure over traffic congestion and to assess the socioeconomic impact of the new investment on this particular area. The research objective of this article is to evaluate the impact of abovementioned investments and answer the following questions:  
* Does the construction of the underpass have a significant impact on traffic improvement? * What are the effects of the works and rehabilitation of the studied blackspot intersection on air pollution? * What tools can be used to analyze traffic and its impact on the urban environment? * What further development directions for optimizing traffic can be taken? Therefore, in order to achieve the above research goals, we conducted a study to examine the spatial and temporal impact of traffic on this intersection and the surrounding areas before, during, and after the investment. Works on the infrastructure began in November 2019 and ended in October 2020. Traffic volumes during and after the infrastructure works were also heavily impacted by the Covid pandemics and by the driving bans imposed during that time.