It's a complex discussion in the Netherlands in which the data protection agency (AP) has a very strict view (they claim it's not allowed) while for example the associated press sees it very different.
There is a key difference between recording vs publishing. There are more restrictions on publishing and an objective assessment needs to be made between the interests of the person in the footage and the general public or publisher.
I would argue that recording the road to collect speed data, not keeping the recording longer than needed and not for example recording license plates, would pass in the Netherlands. Since you're making an assessment between different interests and the is limited privacy impact. Of course assuming this is happening on a public road and not someone's property.
Publishing the recordings instead of just the average speed data would be a very different story, especially if the cars or drivers can be identified.
In practice, no one cares. Cops will even ask you nicely for footage if something went down in your street.
Anyway, best of luck with your project.