It is important to note that these numbers only represent a crude approximation of the effect of the speed limit increase. Seasonality One of the powerful benefits of using structural modeling is that, instead of removing seasonality, the effect of being in a specific month is directly modeled. The strength of the seasonal pattern was one of the most surprising aspects of this analysis. The extent of seasonality is different for each state. Most states typically have a higher number of fatal crashes in August.
Some states have different patterns that have interpretations unique to that state.
For instance, Florida tends to have more fatal crashes in March on its urban interstates. This corresponds to the time most college students are on Spring Break and many choose to drive to locations in Florida. In general, seasonal peaks appear to coincide with peak holiday seasons. Most states do not produce data on vehicle miles traveled, so we cannot adjust the data in a consistent manner for such effects. Though the analysis is by state, it is of interest to generalize the effect of speed limit increases to the nation as a whole.