not part of the federal interstate system of highways which has crossovers or accesses from streets, roads or other highways at the same grade level as such divided highway." Urban Areas are defined as: "an area of fifty thousand population at a density at or greater than one thousand persons per square mile". The highways and transportation commission may raise or lower the speed limit on these highways, but no speed limit may be set above 70 mph on a numbered highway and 60 mph on a lettered highway. Interstate highways have minimum speed limits of 40 mph. Variable speed limits Missouri concluded a two-year experiment with variable speed limits along I-270 around St. Louis. Digital signs had been erected along the freeway as well as additional signs alerting drivers about the use of variable speed limits. The limits will vary between 40 and 60 miles per hour, depending on traffic conditions, and could change by up to 5 mph every 5 minutes. These speed limits, as of January 2012, are now posted as "Advisory Speed Limits". During the closure and major rebuild of I-64 in St. Louis, an additional lane was added to I-44 and I-70, and the speed limit was thus reduced to 55 mph on those roads within the St. Louis County and City. The I-64 construction has been completed, and the extra lanes were removed in 2010. In October 2010, the speed limit was restored to 60 mph on both I-44 and I-70. Exceptions to the statutory limits Most two-lane roads with shoulders have a 60 mph speed limit in Missouri. Two-lane roads without shoulders are usually, but not always, limited to 55 mph. Some improved two-lane highways have a 65 mph speed limit in rural areas, such as: US-54 from El Dorado Springs west to the Kansas state line; from the MO-73 intersection southwest of Macks Creek to near Camdenton; and also along the three-mile-long Mexico bypass. US-63 from Vienna south to Thayer, near the Arkansas state line. Some of this route has three lanes, with the passing lane alternating between northbound and southbound traffic. MO-5 between Camdenton and Lebanon. This road has three lanes, with the passing lane alternating between northbound and southbound traffic. MO-43 from US-54 south to Seneca MO-96 from I-44 west to Carthage Most rural expressways have a 65 mph speed limit, however some have an upgraded speed limit of 70