Final answer:
The period of the traffic flow graph at 1st Ave and High St is equally as long as that at 2nd Ave and Central, because traffic volume does not affect the period of a traffic flow graph which is determined by the cycle of traffic signals or daily traffic patterns.
Step-by-step explanation:
The question asks us to compare the graph of the traffic flow at 1st Ave and High St with the graph of the traffic flow at 2nd Ave and Central over time, specifically in terms of their periods. Given the information provided, there's been a notable increase in traffic at the intersection near 1st Ave and High St due to several factors including the opening of a manufacturing plant, the growth of businesses in a nearby strip mall, and a change in football game traffic routing by a local college. These factors, however, do not directly influence the period of a traffic flow graph but rather its amplitude or the average traffic flow. The period of a graph is the duration of one complete cycle of the graph, which in the context of traffic flow, could be determined by factors like signal patterns or daily traffic cycles.
Addressing the original question, the period of the traffic flow graph for 1st Ave and High St does not inherently change relative to that at 2nd Ave and Central simply because the traffic flow volume is twice as much. Therefore, option 'd' is correct; the period of the graph of the traffic flow at 1st Ave and High St is equally as long as the period of the graph of the traffic flow at 2nd Ave and Central, assuming there are no changes to the frequency of traffic lights or other time-dependent factors that could alter the cycle of traffic flow.