So, I’m going to hop into this conversation and give you this:
https://goo.gl/maps/X1amjZeW7gKUYqyD9See, DDIs are fairly hard to screw up. The design is pretty standard, even for weird alignments. The spur though, can be various sizes, shapes and forms. THAT? That is NOT what a SPUI should look like. The whole POINT of a SPUI is to have 3 phases, Thru, Freeway On, and Freeway Off (unless it’s one of those ones where the service road has a thru movement, but with those you just add a Service Rd Thru phase). With that, coming off the freeway may be without issue, but if you’re turning onto it, we’ll, I’ll leave that as an exercise for the reader.
Honestly, though, this would be a perfect place for a DDI, no significant ramp adjustments are needed, and basically the construction would be:
Make slight adjustments to left-turn on ramps,
Close road overnight for restriping/new traffic pattern,
Install concrete islands directing traffic,
Finalize interchange.
Converting some interchanges to DDIs requires slit more work. And if MD hasn’t put in any DDIs, this could be the perfect test bed.