In November 2006 a report was issued. The study included the points of view of all states affected by this proposal. The short conclusion is that insufficient funding or interest exists to make this a reality any time soon. The study concludes that existing routes in this corridor undergo improvements to access control to approximate the benefit an interstate would provide. The entire report is on VDOT's website - link.

The South Carolina portion of I-99 would essentially follow the US 17 corridor, though clearly it would use SC 31.

The choice of 99 as the number, while fitting the original interstate numbering scheme, would be incongruous given the existence of I-99 in central Pennsylvania. Should this proposal ever go forward, it would be better numbered as 97 (today's 97 should be part of 70 or 83 anyway).

This is not the first time the state of Virginia had put the idea of an Eastern Shore interstate out there. In September 1945, the Virginia CTB requested that the US 13 corridor on the Eastern Shore be added to the Interregional System of Highways (precursor to the Interstate System). See page 12 of Sept 1945 CTB Minutes. This request was ultimately denied. In May 1960 the CTB requested an interstate corridor on the Eastern Shore. See page 55 of May 1960 CTB Minutes for specifics. I did not ever find anything regarding a resolution, e.g. why it didn't get approved.

Thanks to Jeffrey Klugh for pointing out the current proposal to us!