What factor is primarily adjusted in nominal safety assessments?

In nominal safety assessments, the primary focus is on traffic volume over time. This factor is critical because understanding traffic volume helps assess the risks associated with road use and the frequency of potential accidents. High traffic volumes typically correlate with increased opportunities for interactions between vehicles, pedestrians, and other road users, which can lead to a higher incidence of crashes. By analyzing data on traffic volume, safety assessments can identify patterns, predict peak times for accidents, and implement strategies to mitigate risks, such as optimizing signal timing or improving road design.

The other options, while relevant to road safety in broader contexts, do not directly address the primary adjustments made in nominal safety assessments. Crash type provides insights into the nature of accidents but does not inherently adjust for safety on a broader scale like traffic volume does. Optimization of routes focuses on improving travel efficiency rather than safety metrics directly. Vehicle performance looks into the safety characteristics of individual vehicles but is not a direct factor in the assessment of safety on a network level. Therefore, traffic volume stands as the most pertinent factor for nominal safety assessments, allowing for a comprehensive understanding of road safety dynamics over time.