How to import a statewide road centerline file?

You should know that imports are not universally welcome in OSM; imports are not impossible but they are not our usual way of data acquisition. OSM values the work of its volunteer contributors highly, and would not usually replace or preempt such work through a data import. Roughly speaking, an import is a social as well as a technical challenge. You will first have to build a consensus about whether OSM wants the data imported at all, and if that happens, then you will have to think about how to import the data without damaging or duplicating that which is already there. (Simply deleting all existing road centerlines in Arkansas and replacing them would be totally unacceptable to the community.)

On a "whole state" scale, even the preparation for such an import would very likely take several months.

You will find more about the topic of imports if you type "import" in the search box above and read some of the responses, or subscribe to the talk-us mailing list and raise the issue there.

It is usually easier, and certainly less of a contentious issue, if instead of importing a data set into OSM outright, you make it available in a form that our mappers and other volunteers can then use; this might be a WMS from which to trace missing bits or correct those that might be wrong, or it could be a publicly accessible shape file that volunteers can then automatically match against OSM and thereby spot differences. Not every such difference would necessarily point to an error in OSM, and the best way to deal with them would be to survey the area or evaluate suitable aerial imagery to find out who's right.

The biggest problem with a direct import is that major Arkansas roads have seen hundreds of hours of labor invested in creating the routing network, improving alignment, and tagging attributes. Minor roads have been manually improved in selected areas. Any use of the improved state data must preserve the existing work.

One answer is to selectively use the "Replace Geometry" tool from JOSM's Utils Plugin 2 while copying road by road from the official state data loaded into another layer. However, this does not preserve the endpoint connectivity. The official state data tagging would need to be reviewed for proper conversion from shapefiles to conform to OSM standards - for example to remove abbreviations and proper use of upper and lower case.

The problem of how to ease use of updated road network data is quite advanced, and exists at varying levels in many counties across the US. The best solution would be a dedicated editor plugin that allows updating the road geometries one by one while preserving the connectivity, existing tags, and bridges. Some of these updates would require manual intervention to correct the result and cannot be applied on a mass scale.