Slowing down means great savings to a community in many different ways. Slowing down should be both a beneficial side-effect of a well-designed transportation system and also an intentional design element. Self-explaining streets in the form of good design- including narrower, 10' to 11' lanes for motor vehicles, wider 6' to 7' bike lanes, sidewalks and green space- is a postivie way to affect the speeds of motor vehicles in a community.
Considerations in setting community wide speed limits:
The difference in the noise level of a car going 30mph and a car going 40mph is 1000%. In other words, the faster car is ten times louder than the slower car. Research shows that this extra noise adversely affects sleep, play, and learning of people close to busy roadways.
Due to closer following distances, a roadway can move more cars that are traveling 30mph versus 40mph. 30mph is close to the optimal travel speed for maximizing capacity.
Most often, slower travel speeds mean smoother travel, meaning less stop and go traffic. This means less acceleration and braking which is good for your vehicle, saves money and reduces pollution.
A vehicle's speed is a major factor on impacting a community's public space. In general, the faster a vehicle travels the more space it takes up- imagine a 'bubble' of unusable space surrounding a motor vehicle. That bubble gets larger as speed increases.
One study has shown that 17 mph is the maximum speed at which two people can pass each other, recognize one another, and exchange a greeting.