The following highways are numbered 400:

Australia

• Murray Valley Highway

Canada

• Ontario Highway 400

Croatia

• D400 road

Ireland

• R400 road

Japan

• Japan National Route 400

Korea, South

• Capital Region Second Ring Expressway

South Africa

• R400 road in the Eastern Cape

Spain

• N-400 road

Turkey

• D.400, a west–east state road in Turkey running from Datça, Muğla Province to Esendere at the Iranian border.

United Kingdom

• A400 road in London
• B400 road (Chancery Lane)

United States

• U.S. Route 400
• Arkansas Highway 400
• Florida State Road 400
• Georgia State Route 400
• Kentucky Route 400
• Louisiana Highway 400
• Maryland Route 400, a former state highway
• Nevada State Route 400
• New York:
  • New York State Route 400
  • County Route 400 (Erie County, New York)
• North Carolina Highway 400
• Pennsylvania Route 400 (official designation for Pennsylvania Route 380)
• Puerto Rico Highway 400
• South Carolina Highway 400
• Tennessee State Route 400
• Texas:
  • Texas State Highway Spur 400
  • Farm to Market Road 400
• Utah:
  • Road 400 (Cottonwood Canyon Road)
• Virginia State Route 400
  • Virginia State Route 400 (former)