Plastics manholes are designed to withstand traffic loads through a combination of factors. They are made from high-density polyethylene (HDPE) or polypropylene (PP) materials that are strong and durable. The manholes are also designed with ribbing and other structural features that distribute the weight of the traffic load evenly across the surface of the manhole. Additionally, plastic manholes are resistant to corrosion, which helps to ensure their longevity and ability to withstand heavy traffic loads over time.