I read yesterday a review by Mary Beard of a number of interesting publications in the Times Literary Supplement, some of these may have bearing on elements of my thesis so I will have to have a look at them. That’s not what this post is about, however. The works are O’Sullivan Walking in Roman Culture, Laurence and Newsome, eds, Rome, Ostia, Pompeii and Kaster, The Appian Way.
Discussing an essay in Laurence and Newsome, by Kaiser called ‘Cart Traffic Flow in Pompeii and Rome’, in which Kaiser talks about the evidence of Rome being a purely, in Beard’s words, ‘pedestrian city during daylight hours’. This is a common assertion which, as it turns out, (not uncommonly), is backed only with a single piece of epigraphic evidence (from southern Italy). This states that wheeled vehicles are banned in daylight hours (except for Vestal Virgins). Kaiser apparently asserts that the word, plostrum or plaustrum specifically applies to a sort of heavy transport ox-cart only. In other words, this is a heavy-vehicle regulation, not a ban on all ‘cars’. Beard questions the connection between Vestals and ox-carts:
For if he is correct about the plostrum being a heavy utilitarian form of transport, what on earth were Vestal Virgins doing riding on one (as one of the exceptions noted in the regulations makes clear that they did)?
As it turns out I can supply one other connection. I happened to write about the Gaulish sack of Rome and Livy’s Book 5 in my master’s thesis and as soon as I read Beard’s question one answer immediately popped into my mind. There is an episode in Livy 5.40, as the Gauls were about to sack Rome and people were fleeing the impending disaster, the Vestals were fleeing the city on foot, carrying the sacred objects. They were then rescued on a cart by a plebeian (who boots his own family off it to accommodate them!). But was this word ‘cart’ the Latin word plaustrum or some other one, like carrus or currus … and on checking, sure enough, it is (the important part is 5.40.9 and 10, I have highlighted the relevant words):
 Flamen interim Quirinalis virginesque Vestales omissa rerum suarum cura, quae sacrorum secum ferenda, quae quia vires ad omnia ferenda deerant relinquenda essent consultantes, quisve ea locus fideli adservaturus custodia esset,  optimum ducunt condita in doliolis sacello proximo aedibus flaminis Quirinalis, ubi nunc despui religio est, defodere; cetera inter se onere partito ferunt via quae sublicio ponte ducit ad Ianiculum.  in eo clivo eas cum L. Albinius de plebe homo conspexisset plaustro coniugem ac liberos avehens inter ceteram turbam quae inutilis bello  urbe excedebat salvo etiam tum discrimine divinarum humanarumque rerum religiosum ratus sacerdotes publicas sacraque populi Romani pedibus ire ferrique ac suos in vehiculo conspici, descendere uxorem ac pueros iussit, virgines sacraque in plaustrum imposuit et Caere, quo iter sacerdotibus erat, pervexit. (Livy 5.40.7-10)
Here’s the translation from the Loeb in case you don’t have the Latin (or don’t have time to work though it):
 Meanwhile the flamen of Quirinus and the Vestal virgins, with no thought for their own belongings, were consulting which of the sacred things they should carry with them, and which, because they were not strong enough to carry them all, they must leave behind, and, finally, where these objects would be safe.  They judged it best to place them in jars and bury them in the shrine adjoining the flamen’s house, where it is now forbidden to spit; the rest of the things they carried, sharing the burden amongst them, along the road which leads by the Sublician Bridge to Janiculum.  As they mounted the hill they were perceived by a plebeian named Lucius Albinius, who had a waggon in which he was conveying his wife and children, amidst the throng of those who, unfit for war, were leaving the City.  Preserving even then the distinction between divine and human, and holding it sacrilege that the priestesses of his country should go afoot, bearing the sacred objects of the Roman People, while his family were seen in a vehicle, he commanded his wife and children to get down, placed the virgins and their relics in the waggon, and brought them to Caere, whither the priestesses were bound.
OK, just to engage in some wild speculation … what if Livy is here retelling one of those stories are meant to explain some (already) antique practice that he might have witnessed in Rome; say the Vestals and the Flamen Quirinalis being hauled about in plaustra? At any rate it is certainly another link between vestals and such vehicles, that may help with understanding the specific exception given to the Vestal Virgins, for some specific practice.