The difference in a vehicle's city and highway mileage is directly related to the increased acceleration and braking experienced in typical city driving. On highways, the engine is able to settle into an ideal pattern, generating the proper energy for the speed you desire, more so if the speed is kept in all car's optimal range (55-60). When city driving, much of the distance covered is while the car is speeding up from a standstill, a process which consumes far more fuel (and produces more emissions) than driving steadily at one speed.
As mentioned in a previous article, my car has a real time fuel consumption computer which I've referenced for other statistics. In strictly stop-and-go urban driving, the car usually manages about 22-25 MPG. While accelerating, however, that number falls to 10 or below! Consider this: the first light turns green, and the traffic flow begins moving towards the next set of lights, which promptly turns red. At this point, the momentum the car generated is wasted in braking for the next intersection. Therefore, for this area of travel, the vehicle's mileage was likely less than half even the EPA rating!
There is another aspect to consider: idling. As much (or as little) as a non-hybrid vehicle sips fuel when driving, they all get 0 MPG when idling. According to the Canadian Office of Energy Efficiency, the average engine consumes between ¼ to ¾ of a gallon for every hour it is left idling. Considering the average Canadian (American drivers are likely quite similar) leaves their engine idling for 5-10 minutes daily, assuming $2.00/gallon, the car generates about 2.5 pounds of carbon dioxide and directly costs the owner 5-10¢...daily. A good methodology to follow is to turn off the engine if it will be idling for more than about 30 seconds. Modern engines use less gas to restart than they do to operate for 15 seconds. Contrary to popular belief, it is no longer necessary to leave the vehicle idling when cold; it warms up more effectively when being lightly driven.
So it's been established that idling wastes fuel (and generates more CO2), while consistent red lights decrease gas mileage, both contributing to increased costs of vehicle ownership (and increased emissions), but how do they relate?
While the American Federal government is attempting to take a leadership role in reducing emissions and maximizing efficiency, without any detraction to those efforts, it is likely they will be slow to implement. However, development of climate change task forces by local community and county governments have great potential for effecting immediate and substantial change. While traffic elimination is a pipe dream (but something that both "green" and "non-green" citizens would welcome!), one issue that is rarely discussed is traffic light timing.
In most urban and sub-urban regions, nearly all traffic lights on major roads are networked on a central timing system, affected normally by time of day, individual car sensors, and emergency vehicles. The goal of such a system is to promote a smooth flow of traffic in all directions, while maximizing safety in your commute. Additionally, most have a "magic speed" configured, in which, assuming no traffic, one could hit every green light the entire road through. Unfortunately, in many regions, there are certain lights that always seem to be red. It isn't a coincidence, though it might be an accident.
How can you help? Encourage your local climate change task forces (if you have) as well as your city and county commissioners to push for traffic light timing programs. This is probably the first place you've seen it written, but nevertheless: Go green with traffic light timing! By decreasing stop-and-go driving patterns as well as reducing idle time of commuters, localities can make a substantial affect on their own emissions, thereby taking a worthwhile step in their green efforts.
Joseph Winn is the President/CEO of GreenProfit Solutions, Inc. which assists businesses in becoming environmentally responsible. You may view their website at www.greenprofitsolutions.com or e-mail Joseph at jwinn@greenprofitsolutions.com .