There is no inherent physical limitation that prevents a person from riding a bike faster than 30 km/h (18.6 mph). In fact, professional cyclists can reach much higher speeds, often exceeding 40-50 km/h (25-31 mph) during races or time trials.
The maximum speed a person can achieve on a bike depends on various factors, including the rider's fitness level, strength, technique, and the type of bicycle being used. Additionally, external conditions such as wind resistance, terrain, and road conditions can affect a cyclist's speed.
For an average recreational cyclist, maintaining speeds above 30 km/h can require significant effort and training. Factors such as aerodynamics, pedaling efficiency, and power output become more crucial as speeds increase. It can be challenging to sustain high speeds for prolonged periods due to fatigue and physical limitations.
It's worth noting that different people have varying levels of fitness and skill, so what may be difficult for one individual could be achievable for another. With proper training, conditioning, and the right equipment, it's possible for experienced cyclists to exceed speeds of 30 km/h on a regular basis.