Vehicle emissions refer to the pollutants that are released from the exhaust system of a motor vehicle, such as cars, trucks, buses, and motorcycles. These emissions include both gaseous pollutants, such as carbon monoxide, nitrogen oxides, and volatile organic compounds, as well as particulate matter, such as soot and other fine particles.

When vehicles burn fuel to power their engines, they produce these emissions as a by-product of the combustion process. These pollutants can have harmful effects on the environment and human health, such as contributing to air pollution, smog, and climate change. In many countries, there are regulations in place to limit the amount of emissions that vehicles are allowed to produce, and efforts are being made to develop cleaner and more efficient technologies to reduce these emissions even further.

Vehicle emissions in tunnels are a significant concern due to their potential impacts on air quality and human health. The critical design characteristics of vehicle emissions in tunnels include:

1. Ventilation: Tunnels must have adequate ventilation systems to ensure that vehicle emissions are effectively captured and removed from the tunnel. Ventilation systems must be designed to provide sufficient airflow to dilute and remove pollutants.
2. Gradient: Tunnels with steep gradients can cause increased emissions as vehicles must work harder to travel up inclines. Therefore, tunnels should be designed with a low gradient to minimize the amount of emissions produced by vehicles.
3. Traffic flow: The number of vehicles using a tunnel and their speed can affect the amount of emissions produced. Tunnels should be designed to minimize congestion and maintain a smooth traffic flow to reduce emissions.
4. Length: The length of a tunnel can impact the amount of emissions produced as vehicles spend more time within the tunnel. Therefore, tunnels should be designed to be as short as possible, while still meeting their intended purpose.
5. Maintenance: Regular maintenance of ventilation systems and other equipment within the tunnel is critical to ensure they are functioning properly and effectively removing pollutants from the air.

Overall, the design of tunnels should prioritize the reduction of vehicle emissions to protect air quality and public health.