Exegesis:

este hōs ta mnēmeia ta adēla ‘you are like unmarked graves.’ The article ta is generic. Since contact with graves makes ritually unclean, graves used to be whitewashed so as to be conspicuous (cf. Mt. 23.27).

adēlos ‘not clear,’ ‘unseen,’ ‘unmarked.’

hoi anthrōpoi [hoi] peripatountes epanō ‘the people who walked over,’ scil. the graves.

ouk oidasin ‘do not know,’ scil. that they walk over graves.

Translation:

To bring out that the comparison is between the hidden uncleanness of graves and the unknown evil in the hearts of the Pharisees one may specify the object of the last verb, e.g. ‘without knowing how bad/unclean/filthy they (i.e. the graves) are inside,’ choosing for the adjective a term that can be used both of ritual and of moral badness. Further explanation, if required, can better go into a footnote referring to Mt. 23.27f, where the simile is elaborated.

Graves which are not seen, or, ‘are unmarked’; or, ‘graves which people cannot see (for what they are), or, have not marked (as such).’ For grave one may have to use a descriptive rendering, e.g. ‘hole in the ground where a corpse is buried/put,’ or a locative derivation of ‘to inter/inhume’ (Toraja-Sa’dan). In Bali the place where a corpse is buried (either permanently or awaiting cremation) is marked by mounds that rise a few inches above ground level; therefore the phrase has been rendered, ‘mounds that have become flat.’

And men walk over them without knowing it, or, ‘and men who walk over (or, tread upon) them do not know it’; and cf. above. The clause is resultative.

Quoted with permission from Reiling, J. and Swellengrebel, J.L. A Handbook on the Gospel of Luke. (UBS Handbook Series). New York: UBS, 1971. For this and other handbooks for translators see here . Make sure to also consult the Handbook on the Gospel of Mark for parallel or similar verses.