Definition
Post roads are the roads, highways, water routes, and other transportation corridors designated by law as the authorized channels over which the United States mail shall be carried. The term derives from the constitutional power granted to Congress to "establish Post Offices and post Roads" (U.S. Const. art. I, § 8, cl. 7), making it one of the few infrastructure concepts named explicitly in the Constitution.
Distinguish post roads from post routes: a post road is the physical way or corridor designated for mail transport; a post route is the appointed course or prescribed line along which a specific mail service runs. The two terms are related but not interchangeable in legal and administrative usage.
Common Language
Modern common usage (Wiktionary): A road used for transporting mail; historically, a road along which post horses or mail coaches traveled at regular intervals.
Historical common usage (Webster's 1913): A road over which the mails are carried; a mail route.
The common and legal meanings are unusually close here, but the legal meaning carries constitutional weight that the ordinary meaning does not. In law, designation as a post road is not merely descriptive — it is a formal act of Congress conferring federal jurisdiction and, historically, triggering specific legal consequences for obstruction, contract, and rate regulation. A road that mail happens to travel on is not automatically a post road in the legal sense.
Common Confusion
Post roads and post routes are frequently used interchangeably in older sources and general writing, but the legal distinction matters in research contexts. Black's (2nd Ed.) draws the line clearly: the post road is the designated corridor (the where); the post route is the prescribed line of service operation (the how and course). Early federal cases and administrative records may use either term loosely, requiring readers to examine context.
Why It Matters in Research
This term matters primarily because of its constitutional anchor. Congress's power over post roads was historically interpreted broadly — extending well beyond dirt roads to include railways, water routes, and eventually air corridors. Researchers working in 19th-century federal law will encounter post roads in the context of railway mail service disputes, contract litigation between the federal government and private carriers, and early commerce clause jurisprudence.
The Railway Mail Service Cases (13 Ct. Cl. 204), cited in Black's, are a useful entry point for understanding how courts interpreted the post roads power as applied to railroads — a significant expansion of the term's original meaning that generated substantial litigation. Researchers should not assume that "post road" in an 1850s or 1880s document refers only to a physical road; by that period, the term had been extended by statute and judicial interpretation to railways and waterways.
Jurisdictional trap: because Congress designates post roads, there is no state-level equivalent in the strict constitutional sense. State sources will not use this term in the same way. Researchers looking at state road law or turnpike history should not conflate state highway designations with federal post road designations.
The Bouvier's entry is unhelpfully thin, simply redirecting to "Post Office." Researchers relying on Bouvier's alone will miss the substantive legal content available in Black's and the case law.
Historical Dictionary Support
Black's Law Dictionary (2nd Ed.) provides the operative definition and draws the post road / post route distinction with reference to two federal cases. The definition is functional and practical, focused on the administrative and transportation law context in which the term appeared in litigation.
Bouvier's Law Dictionary declines to define the term independently, redirecting to the Post Office entry. This is characteristic of Bouvier's organizational approach to infrastructure and administrative topics, but it leaves the researcher without useful content on the term itself. For historical research purposes, Black's is the controlling dictionary source here.
Neither dictionary engages with the constitutional dimension in depth — neither traces the clause, discusses the debates over Congress's power, or addresses the expansion of post road designations to non-road transportation. Researchers needing that layer should look beyond the historical dictionaries to treatises on constitutional law and the legislative history of postal statutes.
Jurisdictional Note
Post roads are a federal concept rooted in Article I of the Constitution. Designation is an act of Congress, and federal law governs. State law is relevant only at the margins — for example, where state road law intersects with federally designated routes. Non-U.S. researchers should note that the term does not translate directly to postal or transportation law in other common law jurisdictions.
Encyclopedia Cross-Reference
No Law Mind Encyclopedia entry directly addresses post roads or the postal clause. The matched entries (Post-Conviction DNA Testing, Post-Closing Obligations, Right to Silence) share only a prefix and are not substantively relevant.