The question is asking what factors impact noise pollution when the total traffic volume is the same. We can rule out the total traffic volume as a factor because it is constant in this context.

The correct answer is option 1 "% of heavy vehicles." We can deduce this because the percentage of heavy vehicles varies across different locations and there is a correlation between this percentage and the noise pollution level. For instance, location III has the highest percentage of heavy vehicles and also the highest noise pollution level.

Option 2 "noise pollution level and average noise level" doesn`t make sense because the noise pollution level is the variable we are trying to explain.

Option 3 "average noise level and % of heavy vehicles" can`t be correct because the average noise level remains the same for location I and III, but the noise pollution level is different.

Option 4 "indeterminable on the basis of details given" is incorrect because the data allows a clear deduction of the correct answer.