From http://www.texashighwayman.
Exits, entrances, and the ubiquitous frontage roads
As you drive through Texas, you may notice that exits and entrances on many of our Interstates tend to be more frequent than in other states. This is the result of the wishes of a former and long-time director of the state highway department, Dewitt Greer. It was his belief that, given the vastness of the state, Interstates should not just serve "interstate" traffic, but also "interregional" traffic within Texas.
Another byproduct of this philosophy is perhaps the most unique feature of the state's highway system: frontage roads. There are almost 6,800 miles of these along freeways in Texas, more-- much more-- than any other state. The reason for the widespread use of frontage roads in Texas again can be traced to Dewitt Greer. He decided that it was cheaper to build frontage roads to keep access to adjacent properties than it was to purchase those access rights, which is required under state law. This quickly became standard operating procedure at the Highway Department (and subsequently TxDOT) and nearly all of the state's freeways were built under this policy. In June 2001, the Texas Transportation Commission considered a major policy change that would've rocked TxDOT and Texans in general to the core-- frontage roads would not be included in any new freeways unless absolutely necessary. This was a fundamental change and caused quite a stir around the state-- enough of one that the Commission decided, after statewide hearings, to scrap the idea.
There are several ramp configurations used with frontage roads. The "diamond" interchange is pretty standard, with the on- and off-ramps connecting to the frontage roads in the general shape of a diamond relative to the cross street (see illustration below). Sometimes, especially in urban areas, the ramps are reversed in an "X"-interchange with the exit ramp for the next cross street preceding the entrance ramp from the previous cross street. This allows traffic to weave on the frontage road rather than the freeway. Occasionally, if space is limited, both the on and off ramps are built at the same location in a "braided" arrangement (i.e. one ramp passes-over the other).
 Frontage roads with diamond interchanges |  Frontage roads with "X" interchanges |
Speed limits on frontage roads generally range from 60 mph in rural areas to 40-50 mph in urban areas. In urban areas, access roads are one-way in the same direction as the adjacent freeway lanes. In rural areas they are generally two-way. On the frontage road, traffic leaving or entering the freeway has the right-of-way. Yield signs are usually posted, but in many areas they're lacking. Many drivers don't realize that even if there's no sign, they're still required to yield.
Our ubiquitous frontage roads are an oddity unique to Texas. However, within Texas, frontage roads have their own oddity. If you travel through the state, you may notice that people in each of the major cities call their frontage roads something different: in Houston, they're "feeder" roads; in Dallas-Ft. Worth, they're "service" roads; in San Antonio, they're "access" roads. I've heard that they're known as "gateways" in El Paso. The term used in Austin and the state's official term is "frontage" road, which is how you'll see them marked on guide signs statewide, even in the aforementioned areas where the popular semantics differ. You can often tell what part of the state someone hails from just by asking them what they call that road next to the freeway.
It should be noted that frontage roads (or whatever you call them) are not just found on
Interstates-- the freeway segments of many US and state highways also feature frontage roads.
Texans are frequently surprised when they go to other places and find no frontage roads. I often hear people talk about their trips to _____ and one topic that often comes-up is, "They don't have frontage roads on their highways!" Most native Texans don't realize how much of an oddity our extensive frontage roads are. Conversely, many out-of-state visitors immediately notice and comment on our frontage roads. It's usually about 50-50 between thumbs-up and thumbs-down.
Farm-to-market roads
More than half of the state's highway system is comprised of the Farm-to-Market (FM) road system, which also includes Ranch-to-Market (RM) roads and Ranch Roads (RR). Although started in the late 1930s, the system really grew after the 1949 Legislature set aside $15 million annually from the state's General Fund for their construction. General Fund money is no longer used, but the FM system is still paid for entirely by the state. The FM system is the most extensively developed rural highway system in the nation. Its nearly 41,000 miles is more than double the entire state highway mileage of the six New England states combined. There is no difference between FM, RM, and RR routes-- just more Texas highway semantics.
