Well, this might have to do more with the way the human body is designed and how much information we can process in a short period of time. Scientific data has shown that our actual active line of sight window (or the area of our vision which is fully analyzed and can be reacted to instantaneously) is limited to a few square inches in front of you. The rest is what is considered to be the peripheral vision area which we are aware of but do not have too much concentration on. Add to it the tunes playing on the radio, the shifting combination of the depressing the clutch and changing gears and you have a lot of information to handle for the human body to actually process (try this test to see if you are a safe distance from the car in front of you).
At 40 kmh, the average car (with stock brakes) can stop reliably in about 5 car lengths which translates to around 50 or 60 feet. This might sound quite far but this is true. Now back to the speed limit thingy, well these are placed due to many considerations such as the type of road surface, the amount of traffic that is to be expected, and the surmised fuel savings and so on and so forth. The one about speed in relation to fuel consumption is a good one, for at 80 kmh, your vehicle is running in balance with it’s weight and the momentum or it’s forward thrust that the engine is running efficiently burning only enough fuel to maintain that speed no more and no less. Now with most carburetor type vehicles that would be a good idea for these vehicles do not have computer brains like the newer models of cars which seem to have technology crammed into each and every corner.
The ECU (Electronic Control Unit) or computer of newer cars has the necessary programming to obtain information from a bank of sensors on the engine (temperature, idling speed, revolutions, amount of air entering the engine and many more) that allows it to adjust the performance of its parts to provide or attain maximum efficiency while maintaining fuel economy. Depressing the gas pedal on a carb-type vehicle and you pour gas into the engine even if it doesn’t need it sending some into the overflow line onto the street. Newer cars have recovery lines which send excess fuel back into the fuel delivery system or the gas tank for re-use. The speed limit is the determined safe speed to maintain necessary distances for braking allowing you enough time to react properly to the events that unfold such as in emergency cases in front of you. The human body reacts quite fast (in milliseconds actually which is the time your eyes see the information, sends it to the brain, processes it and comes up with the necessary response and sends the nerve impulses to the necessary muscles on your limbs for you to react) but at high speeds, that might not be enough. Why race down a street which is only a few blocks long? Just ease on the gas pedal and accelerate gradually for it not only saves fuel, it also saves on the wear and tear on other parts of the vehicle(brakes, suspension and other parts). Enjoy the trip and not the ride for it allows you to ponder about better things than the darned traffic jam ahead of you.