## 7 Conclusions and Future Research Needs
### Conclusions  
The most robust and consistent finding is that certain waste plastics--primarily LDPE, HDPE, PP, and to a lesser extent PET--can substantially improve the mechanical properties of asphalt binders and mixtures. The enhancements include superior rutting resistance, elevated softening point, enhanced fatigue life, and extended service life. Optimal results are often attained when plastic content is meticulously regulated (3-8% in wet methods; up to 10% in dry), and when plastic particle size and compatibility correspond with binder and mixture requirements.  
The method of incorporation plays a critical role. Wet-processed plastic-modified binders typically exhibit enhanced rheological properties, improved compatibility, and diminished long-term environmental hazards, especially in mitigating MP emissions. Dry-processed applications are more straightforward to scale and permit greater plastic reuse, although they may demonstrate less uniform dispersion and increased hazards of MP release due to traffic abrasion. Hybrid systems and functionalized plastic modifiers are emerging as viable strategies to balance performance and sustainability.  
LCAs consistently demonstrate that plastic-modified asphalt pavements can diminish greenhouse gas emissions, raw material consumption, and maintenance frequency, particularly when utilizing locally obtained waste plastic and minimizing transport and preprocessing impacts. Moreover, the incorporation of polymers inside the asphalt matrix can reduce MP emissions when designs are optimized.  
However, several critical concerns remain. Although the modification of waste plastic improves mechanical qualities like stiffness and rutting resistance, these enhancements may lead to performance contradictions, including increased brittleness and reduced fatigue life at higher dosages. Fuming studies underscore the possibility for elevated VOC and PAH emissions when unsuitable plastics (e.g., rABS, PVC) or excessive processing temperatures are employed. Certain plastics, like PET or HDPE, which offer enhanced mechanical advantages, may necessitate higher processing temperatures, hence escalating future emissions and environmental hazards. Similarly, whereas LDPE enhances flexibility and workability, it may present issues associated with microplastic release if not fully integrated. The danger of long-term MP release differs among studies and is affected by incorporation method, plastic type, and environmental exposure. Furthermore, field validation is limited: most existing data originate from laboratory studies or short-term trials, without adequate large-scale, multi-climate monitoring.  
Feedstock variability such as differences in polymer crystallinity, contamination, and molecular weight continues to challenge reproducibility, and although most LCAs indicate net environmental advantages, certain trade-offs (e.g., escalating surface roughness with time, emissions from transportation) may undermine benefits if not carefully managed through integrated pavement design.  
In conclusion, integrating waste plastics into asphalt pavements presents a significant opportunity to improve road performance while advancing sustainable waste management objectives. To safely and effectively capitalize on this potential, future advancements must concentrate on standardizing material specifications, perfecting processing techniques, confirming long-term field performance, and rigorously evaluating environmental impacts through unified testing and monitoring frameworks.