To explain it quickly: You know Heteronormativity, the fact that many people are hetero and assume others are too. This is a similar normativity. Many people (depends on where you live yadda yadda) always drive everywhere and never consider a world where cars are not their only/primary way to get around. They will fail to imagine cities where cars are not as important or common, because they associate transport=car. This is harmful, because cars are in fact very inefficient for most tasks and have pretty bad side effects such as taking tons of valuable space in a city or noise or pollution.