This setting affects the preference Google uses across its services and may influence the default in Maps.
Open your phone's System Settings app.
Search for Google Assistant or just Assistant.
Go to Assistant Settings (it might be under "Other Google apps" or similar).
Search or look for Transportation or Getting around
You may need to have the Google Assistant app installed.
In the Google Assistant app, tap your Profile Picture or Initial in the top right.
Go to Settings.
Find Transportation.
Set your preferred mode to Car or Driving.
You could also delete and reinstall your Google maps.
Because the app is designed to guess your mode, the most consistent way to force it to driving is to always tap "Directions" first, and then choose "Start" afterwards