Yes it can be a sin.
However I think it is reasonable to judge that in regards to the lawgiver (the lawful authority) that he/she knows that customarily persons go say around 5 or so over the posted limit…so they intentionally post the limit lower than they intend people to go…hence the intention of the lawgiver(which is part of the moral aspect of the law) is that one stay around that limit say within a small reasonable amount… miles per hour…(depending on various things…such as speed limit or road or weather etc) …and one usually finds that police do not enforce the limit as posted…but give that customary room…
of course one needs to make a good judgment of conscience…and avoid danger etc. But I think that this is some reasonable thoughts on the matter…
This is not to encourage going faster than need be…but one should also avoid scruples.
(though say school zones and really low speed limits should be that speed or under…think “kids”…that keep strictly!..and more serious fines can mean they are mean it more strictly…and neighborhoods with 25…keep to that…)
As to serious sin – yes it can be serious or I would think venial.
Example of serious would be:
CCC 2290 - The virtue of temperance disposes us to avoid every kind of excess: the abuse of food, alcohol, tobacco, or medicine. Those incur grave guilt who, by drunkenness or a love of speed, endanger their own and others’ safety on the road, at sea, or in the air.
This too will be of interest:
vatican.va/roman_curia/pontifical_councils/migrants/pom2007_104-suppl/rc_pc_migrants_pom104-suppl_orientamenti-en.html
I should note too …remember …what Europeans consider a strict observance is not the
same thing as what Americans do…we have different approaches to law…etc
One needs to consider very much the road conditions…weather…etc…
but I think still that the intention of the law giver comes into play as I noted
and Jimmy Akin’s note here regarding
jimmyakin.com/2006/08/poker_legamoron.html “legemoron” where the lawgiver is expectingvariance from the limit…and the limit is there to enforce especially if ones is driving dangerously …too fast etc…and it keeps people hopefully around a safe speed…
and as we can see from the quotes above…it can be even very grave! We need to take good care…keeping say good distance (thats right…please do not tail gate!) etc…