Although many people believe that Samuel Johnson said "The road to hell is paved with good intentions," he shouldn't get credit for this one.
Johnson said something close, but he was following in others' footsteps. In Boswell's Life of Johnson, in an entry marked April 16, 1775, Boswell quotes Johnson as saying (on some other occasion), "Hell is paved with good intentions." Note, no prefatory "the road to..." Boswell's editor, Malone, added a footnote indicating this is a 'proverbial sentence,' and quoting an earlier 1651 source (yet still not in the common wording).
Robert Wilson, in the newsgroup alt.quotations, provided two other sources prior to Johnson. John Ray, in 1670, cited as a proverb "Hell is paved with good intentions." Even earlier than that, it's been attributed to Saint Bernard of Clairvaux (1091-1153), as "Hell is full of good intentions or desires." Just how it got to the road to Hell being paved this way, and not Hell itself, I don't know.