After several minutes on the West Highway, they turned off onto a dirt road.

[alpacinou]

What can I use instead of "turn off onto" in this sentence?

After several minutes on the West Highway, they turned off onto a dirt road.

 

[5jj]

they turned right/left onto ...
they took a side turning onto ...

 

[probus]

In AmE we often use "take an (or the) exit" in such contexts. When referring to freeways (motorways in BrE) that is invariably the usage.

 

[Skrej]

If you want slang, you could use something like 'hung a left/right onto X'.

You could also use something like 'veered'. 'Ventured' doesn't directly mean 'turn', but it's an understood turn in that context.