20mph Speed Limits: Fuel Efficiency Debate in Urban Areas
While higher speeds are generally more fuel-efficient, studies suggest 20mph zones may use less fuel in stop-start urban traffic due to reduced acceleration energy.
The increasing implementation of 20mph speed limits in urban areas across the UK has sparked debate among drivers about the impact on fuel efficiency. While many perceive lower speeds as inherently less economical, research indicates that the reality is more nuanced, particularly in congested city environments.
Fuel Consumption at Lower Speeds
Traditionally, engines are most fuel-efficient when cruising at speeds between 55mph and 60mph. At lower speeds, such as 20mph, drivers often find themselves in lower gears, leading to higher engine revolutions per minute (RPM). This can increase fuel consumption compared to maintaining a steady 30mph. For petrol engines, driving at a partially open throttle to maintain 20mph can restrict airflow, further reducing miles-per-gallon performance.
However, this analysis typically applies to consistent cruising speeds. In dense urban areas, characterized by frequent stops, starts, traffic lights, and junctions, drivers rarely maintain a constant speed. A 2023 study by Future Transport London (FTL) analyzed the impact of 20mph versus 30mph limits on various vehicles over a simulated 2.5-mile journey through London. The study concluded that in stop-start traffic, 20mph zones are generally more fuel-efficient.
The Role of Acceleration
The FTL study found that the energy required to accelerate vehicles up to the speed limit was the largest contributor to fuel consumption in urban conditions. For a petrol Ford Focus, it was calculated that driving at a consistent 30mph needed to be sustained for approximately 0.56 miles to offset the additional energy required to accelerate to that speed compared to 20mph. In urban settings, where such continuous speeds are seldom achievable, traveling at 20mph proved more efficient.
For instance, the petrol Ford Focus demonstrated an average fuel efficiency of 37mpg in a simulated 20mph zone, compared to 27.4mpg at a 30mph limit over the 4km test route—a 35% increase in efficiency at the lower speed. Larger vehicles like a Mazda CX-9 petrol SUV and a BMW X5 diesel SUV required even longer distances at a consistent speed for 30mph to become more economical.
Petrol vs. Diesel and Driving Style
The study also examined the differences between petrol and diesel engines. While diesel engines contain more energy per unit, the fuel consumption at 20-30mph speeds was found to be nearly the same. Regarding driving style, the FTL report indicated that while conservative driving (using higher gears at lower RPMs) does save fuel, its impact is nominal compared to the energy expenditure of acceleration in urban traffic. For a Ford Focus, conservative driving in 30mph limits yielded only a 4.7% fuel saving over balanced driving.
Emissions and Other Considerations
Beyond fuel efficiency, 20mph limits are primarily implemented to reduce road casualties and encourage walking and cycling, which can indirectly lower local transport emissions. Independent testing specialist Emissions Analytics noted that while combustion engines are generally less efficient at very low speeds, the process of repeatedly accelerating in stop-start traffic, common in cities, generates significant emissions. Therefore, in congested urban environments with frequent stops, a lower speed limit can reduce overall emissions.
However, FTL suggests that 'environmentally conscious' authorities should implement 20mph zones strategically. Stretches of road longer than 0.5 to 0.8 miles without junctions or traffic calming measures might see increased localized pollution if speed limits are reduced, as cruising at 30mph is more economical on such roads.
Broader Impacts
Road safety organizations advocate for 20mph limits due to the correlation between lower speeds and reduced severity of accidents. From an insurance perspective, fewer and less severe collisions can lead to lower claims costs, potentially benefiting drivers through reduced premiums. The AA also noted that a blanket 20mph limit might dilute the perceived importance of lower speeds in areas with high community benefit, such as near schools or medical facilities. A significant challenge highlighted is the enforcement of these lower limits, with calls for increased police presence to ensure compliance.