In the diagram below, I have charted the Dow Jones Transportation Average (blue) and the Nymex Crude Oil tracker (orange). This is stretched over the past year. As can be seen, from October of last year until March, any increase in crude oil reflected as a decrease in Transportation Average. If the primary underlying cost to transportation businesses is gasoline, then it can reasonably be expected that transportation profits, and indirectly stock prices, would decrease as the price of oil increases.
However, it seems through April and May, rising crude prices did not seem to have this correlation suggesting that rising prices sometimes do not have as great an impact as expected. From June, the expected patter returned, with minor gains in the Transportation average as crude prices started a decline in July. Although crude prices have now fallen significantly, the Transportation average has dropped off considerably. This suggests something bigger than crude prices hurting expectations of transportation industry performance--I believe this reflects the overwhelming cloud of the current financial crisis.
If demand drops considerably, no matter how low crude prices are, transportation company performance and growth will suffer. Likewise, in April and May, it could have been that the broader fundamentals in transportation were strong--efficiency, pricing strength, multi-modal utilization, etc--lowering the impact of high crude prices.
Of course, I am not a professional analyst, but I think following the Transportation Average trends relative to other index trends provides some additional insight as to what condition the economy is in, what it is telling us.