Exit 5 on I-495 would be Exit 10, Exit 3 on I-95 would be Exit 6, etc.
For grins & giggles; I did a couple of quick GSV mile marker observations. You're not too far off in saying that I-95's & I-495's current numbers would double, in most instances, if converted.
Exit 5 off I-495 would be Exit 11. MM 11.0 is located within the overall interchange footprint.
Exit 3 off I-95 would be Exit 7 due to the DE 273 overpass is located beyond MM 6.5.
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For those that are wondering, DE 1's MM 102.0 is located just south of the I-95 overpass. So DE 1's Exit 165 would indeed be Exit 102 if such was (re)converted.
Exit numbers can vary, and often are within the MP, not rounded. There's no actual MUTCD guidance on this from what I can tell, and also depends on their relative locations to each other. Using 295 in NJ as an example, Exits 34 (MP 34.8), Exit 36 (36.9) and Exit 40 (40.6) are examples of such. When you get to Exit 43 (mp 43.1), it retains the MP reference. I-195 in NJ has a mix, especially between Exits 1 and 5. Once you get to the NJ Turnpike and continue east, they appear to remain within the milepost number itself. Exits 21 and 22 are nearly 2 miles away from each other: Exit 21 is at MP 21.05; Exit 22 is at 22.95!
How is 476 numbered? I thought it was in threes but after exit 9 it goes to 13?
When the Blue Route portion of I-476, between I-95 & I-76 first opened circa Dec. 1991; the interchanges were sequentially numbered. When PA adopted mile-marker-based interchange numbering circa 2000. I-476's interchanges were renumbered with respect to where its mile markers were. Although IMHO, the PA 3 interchange should've been renumbered to Exit 8 rather than 9 due to the actual PA 3 crossing occurs at/around I-476's MM 8.4. PennDOT, in this case, rounded upward rather than downward.
OTOH, US 30 crosses I-476 at/around MM 13.1; so that interchange being Exit 13 is completely logical.
As J&N mentioned, there's no hard & fast rule for when to round up or down with respect to the mile markers. As a result, there's bound to be some inconsistency for interchange numbering along a particular highway. In the case of PA's conversion; at least a couple of interchanges had
fudged conversions in order so that it's number didn't require a change.
I-476's (Northeast Extension) Exit 31, by right should've changed to Exit 30 due to where PA 63 crossed I-476 but since the
old number was only off by one; the PTC let the old number stand.
PennDOT also fudged a couple of numbers along I-95 in Delaware County so that no change to the existing interchange numbers was required for the southernmost 10 miles in PA.