A vehicle that is maintained to a sufficiently high standard to be used legally on a public road is road-worthy. The quality that it possesses is that of 'road-worthiness', not 'road-worth'.



Steve, your second definition conflates two separate ideas:

2. characterised by good intent but lacking in humour or imagination.
"worthy but tedious advice"
.............
.............
It sort of means competent enough but just dull and uninspiring.


So,
"worthy": characterised by good intent; (sort of means) competent enough
but
"tedious": lacking in humour or imagination; (sort of means) just dull and uninspiring.



DC