KE = ½mv² shows that kinetic energy is directly proportional to the mass of an object and proportional to the square of its velocity. This means that doubling the mass doubles the kinetic energy, but doubling the velocity quadruples the kinetic energy.
The kinetic energy of an 1800 kg car traveling at 30 km/h is about 62.5 kJ
The kinetic energy of an 1800 kg car traveling at 50 km/h is about 173.6 kJ
The kinetic energy of an 1800 kg car traveling at 70 km/h is about 340.1 kJ...
If your only consideration is getting to your destination very few minutes sooner with complete disregard for other peoples health it makes perfect sense, but cars are dangerous and at anything but completely isolated roads it makes sense to lower speed limits since the average speed wouldn't drop significantly while improving safety for everyone.