# Dynamic Low-Emission Zones for Urban Mobility: A Systematic Review
Older gasoline and diesel cars, particularly those not meeting Euro 4 or higher (depending on regional regulations), face access restrictions, daily charges, or outright bans. Electric and hybrid vehicles are typically unaffected and may benefit from incentives such as free access or preferential parking. Trucks and buses contribute significantly to urban emissions due to their high fuel consumption and long operating hours. Many LEZs impose stricter regulations on diesel trucks and buses, often requiring compliance with Euro VI standards [4]. Public transit fleets are increasingly switching to electric or low-emission alternatives to meet the LEZ criteria. Although many cities impose fewer restrictions on motorcycles due to their lower emissions per vehicle, older two-stroke engine models, which release higher levels of pollutants, may be subject to LEZ limitations. Given their high mileage, taxis and ride-sharing vehicles are often required to meet stringent emission standards, with some cities mandating the adoption of hybrid or fully electric fleets. Delivery and Light Commercial Vehicles (LCVs) play a crucial role in urban logistics, and, thus, many LEZ regulations offer transition periods or exemptions for small businesses. However, cities increasingly incentivize the adoption of electric vans or alternative fuel vehicles. Ambulances, police vehicles, and other essential services often receive exemptions, although many municipalities encourage their gradual electrification or conversion to cleaner technologies  
Stockholm pioneered Europe's first Environmental Protection Zone (EPZ) in 1996 [5], targeting heavy-duty vehicles and buses powered by compression ignition engines (primarily diesel). London followed in 2008 [6], launching its LEZ as part of a broader \"Transport for London\" strategy aimed at improving air quality by restricting high-emission diesel trucks and buses. London's LEZ now covers most of Greater London, enforcing strict emission standards and imposing tolls on noncompliant vehicles. Today, there are more than 320 LEZs in the European Union, with projections that this number will exceed 500 by 2025 [7]. As cities increasingly adopt LEZs, understanding their effectiveness, especially when enhanced by dynamic real-time adjustments, is essential for refining these zones to better balance air quality and urban mobility. In the classic literature on routing and dynamic traffic assignment, special emphasis is given to the concept of optimum in its various aspects: system optimum (SO), user equilibrium (UE), and its many variants [8; 9]. Interestingly, when dealing with an LEZ, it seems that the system optimum criterion prevails considering the criteria of the environment,emissions, and pollution parameters, leaving user balancing criteria in the background. It seems reasonable to raise questions such as the following:  
* What should be the optimal extent of an LEZ at each moment in time depending on the traffic condition and quality parameters, and what would be its impact? * What is the penalty generated by an LEZ to the individual mobility of the citizen? * Should the topology of such an LEZ be considered depending on its impact to mobility? * Should an LEZ be active in a certain area if the thresholds are not met? * Can better LEZ topologies be achieved for the quality thresholds raised with less impact to the citizen? The establishment of an LEZ raises legal issues derived from the rights to use roads and also political problems facing promoters and detractors. A dynamic LEZ would be a reasonable proposal to help manage these concerns. The aim of this study was to locate the previous work on dynamic LEZs and make comparisons between them. Another aim was also to identify mechanisms, policies, and strategies that would be feasible to develop such models. One of the great challenges we faced with the study was that there were no studies on real LEZ low-emission zones that applied dynamic criteria over time, considering the state of the roads or the actual air quality.