你说的 by bike, by bus, on foot 等完全可用作表语。

这类用法的句子主语通常是表示方式或旅程的名词。如：

The best way to get there is by bus. 到那里最好的办法是乘公共汽车去。

The quickest way to travel is by air. 外出旅行最快的办法是坐飞机。

In the past, the only way to travel long distances was by sea. 从前远行的唯一办法就是乘船。

Philip left by car but most of his journey was by plane. 菲利普离家时是坐汽车走的，但他的大部分旅程都是坐飞机。

The quickest means of travel into the jungle is by canoe. 到丛林旅行最快捷的方法是乘小划子进去。

The quickest means of travel is by plane. 最快的交通工具是飞机。

The last lap of their journey was by ship. 他们旅程的最后一段是坐船。

The best way to explore the countryside is on foot. 在农村考察，步行是最好的方法了。

注意，你绝不可以受汉语表达的影响像下面这说：

误：I was by bike to come here.

误：I was by plane to travel.

误：We are on foot to go to school.

应改为：

误：I came by bike.

误：I travelled by plane.

误：We go to school on foot.

顺便提一下，下面这类表达 by 短语也用作表语：

Admission is by ticket. 凭券入场。

Admission to any university is by examination. 上大学要通过考试的。

The method of choosing the government is by election. 政府是由选举产生的。

还有这样的表达（可视为by前面省略了不言而喻的过去分词）：

This music is by Beethoven. 这首乐曲是贝多芬创作的。

The picture is by Picasso. 这幅画是毕加索画的。