I was struck by that quote, too. She didn't say "we weren't going fast at all" or "we were going rather slowly"... she said "we weren't going very fast, considering". That tells me they were going pretty fast (keeping in mind that she has reason to downplay their speed). Also the reports are that it was a windswept, rainy night. Any car approaching a checkpoint near a U.S. base, on that dangerous stretch of the Airport Road, in those conditions at anything but a cautious crawl is courting danger.
Something else...
<< An official said they had not given any news of Sgrena’s release to the American embassy in Baghdad, or to US military commanders >>
...makes me think the Italians were trying to do this all secretly, to spirit her back to Italy to cover up the ransom payment; she would just magically reappear back home.
No wonder they didn't want to stop at a checkpoint.