这是一个很值得探讨的问题，看着很简单，实际上水很深。

这里面涉及到一般现在时和现在进行时用法的细微区别，也涉及到when引导的从句时态。

1. You should be careful when you cross the road. 

2. You should be careful when you are crossing the road.

第一句在没有上下文的情况下，一般理解为一般现在时的”习惯”，可以脑补一个场景：一位慈祥的母亲在通识课上告诫自己的孩子一些要注意的地方。

但是也可以理解为表将来，when引导的将来的行为用一般现在时替代（考试常考），因为是个孤立的句子，如果加个时间状语的话表示就很明显了：

You should be careful when you cross the road tomorrow.(想像一个场景体会一下)

第二句很值得探讨。现在进行时的用法就那么四种。针对这个第二句，有几种理解：

1.正在进行中（Such situations may not be happening at the moment of speaking），这个好理解。

举例：The telephone always rings when you are having a bath.

2.表将来，但是该句并没有时间状语，所以不明显，同样，加个tomorrow：

You should be careful when you are crossing the road tomorrow.（明显表将来）

再比如：

Come and see us next week if you’re passing through London.

I’ll think of you when I’m lying on the beach next week.

3. Repeated actions：The present progressive can refer to repeated actions and events, if these are happening around the moment of speaking.

You should be careful when you are crossing the road .（想像一个场景体会一下）

再比如：

She’s at her best when she’s making big decisions.

You look lovely when you’re smiling.

You only seem to come alive when you’re discussing your work.

Whenever I see Graham, he's wearing a tracksuit.

Our tent represents home for us when we are camping.

这个问题是另一个问题，简答之：

Repeated actions not only around the moment of speaking: simple present.

Repeated actions around the moment of speaking: present progressive.

回到你提出的问题，“是否可以理解为当表达习惯意义时持续性动词的一般体可以替代进行体？”我认为一般都不行，一是要看具体上下文；二是要注意这两种时态表示出来的意义始终都会有细微的区别，上面我已经列出了这些细微的区别，你自己细细体会。