Stoma density and pollution
The hypothesis that the stomata density of pine tree leaves taken from trees found in locations with a higher concentration of traffic will be lower, is correct.
Heavier traffic flow will result in a higher level of pollution and higher levels of emissions of carbon dioxide. The presence of more carbon dioxide may stimulate plant growth. But the plant will also require more nutrients, sunlight and water for the growth to be accelerated. Unless these resources are readily available, the plant will need less stoma.
Try to repeat this science fair project by taking leaf samples from different types of trees.
Plant transpiration - http://www.juliantrubin.com/encyclopedia/botany/transpiration.html
Stoma - http://en.wikipedia.org/wiki/Stoma
Carbon dioxide and plant response - http://www.concord.org/~btinker/gaiamatters/investigations/stomata.html
Basic safety requirements