Below are two example videos showing the time taken to cross March Town at speeds driven up to 20mph, and up to 30mph. These are just typical days with light traffic.
March Town at a Maximum Speed of 20 mph
In this first video at speeds of up to 20mph, there is little traffic and the journey is quite relaxed. The time taken to complete the journey was 05:45.
March Town at a Maximum Speed of 30 mph
In this second video at speeds of up to 30mph, there is also little traffic, the journey is was more stressful with more accelerating and braking due to trying to drive as fast as possible, while remaining safe and within the law. The time taken to complete the journey was 05:49.
I drive this route regularly and at various time of the day with various levels of traffic. The shortest possible time to drive this route legally and safely is around 5 minutes with no traffic, no hazards, and no waiting for traffic lights. However it may take anything up to 15 or even 20 minutes due to prevailing traffic conditions.
In the above videos, driving at a maximum of 20mph was slightly quicker than when driving at a maximum of 30mph, but this was due to slightly longer waiting times at traffic lights (in this case caused by reaching them earlier). Therefore waiting for traffic lights can have a greater impact on extending journey times than reducing upper speed. In this case any time saved by reaching 30mph was negated by waiting at traffic lights.
Driving at a maximum of 20mph was also, safer, more courteous to other road users, less intimidating to cyclists and pedestrians, less stressful to the driver and with less braking and accelerating it returned better fuel economy, less wear and tear on the brakes, as well as lower emissions. For these reasons it makes more sense to drive at an upper speed of 20mph in urban areas than an upper speed of 30mph, whether there is a 20mph speed limit in place or not.
Fuel Economy (as determined by the vehicle’s on board computer system)
Video 1 – Maximum Speed of 20 mph – average 53.9mpg
Video 2 – Maximum Speed of 30 mph – average 42.2mpg
While there is a common belief that cars are more fuel efficient at 30mph than they are at 20mph (and there may be some truth in that when speeds are constant), where there is an element of accelerating and decelerating such as driving in an urban environment, limiting the upper speed to 20mph returns better fuel economy and therefore also reduces emissions. At least that was the case in our example. When you consider that both of these journey times were almost identical (and therefore the engine was running for the same amount of time) but a maximum speed of 20mph reduced fuel consumption by 27%, this alone makes a good argument for driving at 20mph in urban areas whether there are speed restriction in place or not.
These comparisons are not intended to be absolute like-for-like. Since traffic conditions (and other factors such as weather) are different every day, it is difficult to make such comparisons without extensive and controlled data. However what it does show is that, faster is not necessarily any quicker (or more fuel efficient) and that the main component for longer journey times for urban journeys in light traffic conditions is the waiting caused by light controlled junctions rather than any reduced upper speed. When the traffic is heavy, the journey time is considerably lengthened and reaching the upper (legal) speed of 30mph can only be achieved for very short bursts, which is unsafe for other road users, has little effect for time saving, and dramatically increases the fuel consumption and emissions.