Dumb question, but is that 498 seen on the reassurance shields mileage? Is that a milepost for US 59?
That is properly referred to as a reference marker, but is often called a mile marker. They don't exactly measure mileage, but are generally placed every two miles, showing only even numbers. They're often placed on alternating sides of the road, so there's one on each side every four miles. The small number plate is placed on both sides of the sign post (on undivided roads). They're used on non-Interstate routes, with numbers increasing west-to-east or north-to-south. A marker is generally placed at a county line showing the next even number in the direction of increasing numbers, regardless of the distance from the previous marker, making the difference in numbers not equal to mileage across counties. The number at the beginning of a road is based on the distance of the beginning of the road from the north end of the state for north-south routes, and at the west end of the state for west-east routes, so the numbers are larger than the actual length of the road. Where they fall near a junction, they're placed at the proper location in addition to the usual reassurance shield. There may be cases in which a marker is located near enough to the location for a reassurance shield that a single assembly serves both purposes. I don't know of any.
To see the locations of (nearly) all of them, see the statewide planning map, and select "Markers" in the Map Overlays section. Note that Interstate highways have actual mile markers placed every mile, but only even numbers are shown on the map.
http://www.txdot.gov/apps/statewide_mapping/StatewidePlanningMap.html