TM data has no way to say that the 3 I-35's and 2 US 20's are the same route in some meaningful way, but pairs like the pairs of I-84, I-86, I-87, I-88, are not.
If we add more data, the data can do that - all you'd need some other .csv file. A simplish reworking usai_con.csv gives us usai_rel.csv below. I've only done the 2dis in the lower 48, as that gives examples of everything while keeping it both concise, but complete (rather than just having random rows out of it), based on the following notions: disjointed routes (a lack of construction, suffixed routes, ferries, etc) are merged, along with branching routes (NJ I-95Sec), bannered routes* (I-5Trk, though if I did the 3dis then the two I-895Spr routes) and suffixed routes (YMMV as to whether they count here). I've got I-74s as well as 76s, 84s, 86s, 87s and 88s, but only one I-69 as there's actually a desire to link the segments of that route.
RelatedRouteGroup;GroupName;roots
I-2;;tx.i002
I-4;;fl.i004
I-5;;ca.i005,or.i005,wa.i005,ca.i005trknew
I-8;;ca.i008,az.i008
I-10;;ca.i010,az.i010,nm.i010,tx.i010,la.i010,ms.i010,al.i010,fl.i010
I-11;;nv.i011
I-12;;la.i012
I-14;;tx.i014
I-15;;ca.i015,nv.i015,az.i015,ut.i015,id.i015,mt.i015
I-16;;ga.i016
I-17;;az.i017
I-19;;az.i019
I-20;;tx.i020,la.i020,ms.i020,al.i020,ga.i020,sc.i020
I-22;;ms.i022,al.i022
I-24;;il.i024,ky.i024,tn.i024,ga.i024,tn.i024cha
I-25;;nm.i025,co.i025,wy.i025
I-26;;tn.i026,nc.i026fau,nc.i026,sc.i026
I-27;;tx.i027
I-29;;mo.i029,ia.i029,sd.i029,nd.i029
I-30;;tx.i030,ar.i030
I-35;;tx.i035,tx.i035e,tx.i035w,tx.i035gai,ok.i035,ks.i035,mo.i035,ia.i035,mn.i035far,mn.i035e,mn.i035w,mn.i035
I-37;;tx.i037
I-39;;il.i039,wi.i039
I-40;;ca.i040,az.i040,nm.i040,tx.i040,ok.i040,ar.i040,tn.i040,nc.i040
I-41;;il.i041,wi.i041
I-43;;wi.i043
I-44;;tx.i044,ok.i044,mo.i044
I-45;;tx.i045
I-49;;la.i049,la.i049bel,ar.i049tex,ar.i049,mo.i049
I-55;;la.i055,ms.i055,tn.i055,ar.i055,mo.i055,il.i055
I-57;;mo.i057,il.i057
I-59;;la.i059,ms.i059,al.i059,ga.i059
I-64;;mo.i064,il.i064,in.i064,ky.i064,wv.i064,va.i064
I-65;;al.i065,tn.i065,ky.i065,in.i065
I-66;;va.i066,dc.i066
I-68;;wv.i068,md.i068
I-69;;tx.i069c,tx.i069e,tx.i069erob,tx.i069w,tx.i069,ms.i069,ky.i069,in.i069whe,in.i069,mi.i069
I-70;;ut.i070,co.i070,ks.i070,mo.i070,il.i070,in.i070,oh.i070,wv.i070,pa.i070,md.i070
I-71;;ky.i071,oh.i071
I-72;;mo.i072,il.i072
I-73;;nc.i073
I-74;Iowa - Ohio;ia.i074,il.i074,in.i074,oh.i074
I-74;North Carolina;nc.i074,nc.i074hig,nc.i074pem
I-75;;fl.i075,ga.i075,tn.i075,ky.i075,oh.i075,mi.i075
I-76;Colorado - Nebraska;co.i076,ne.i076
I-76;Ohio - New Jersey;oh.i076,pa.i076,nj.i076
I-77;;sc.i077,nc.i077,va.i077,wv.i077,oh.i077
I-78;;pa.i078,nj.i078,ny.i078
I-79;;wv.i079,pa.i079
I-80;;ca.i080,nv.i080,ut.i080,wy.i080,ne.i080,ia.i080,il.i080,in.i080,oh.i080,pa.i080,nj.i080
I-81;;tn.i081,va.i081,wv.i081,md.i081,pa.i081,ny.i081
I-82;;wa.i082,or.i082
I-83;;md.i083,pa.i083
I-84;Oregon - Utah;or.i084,id.i084,ut.i084
I-84;Pennsylvania - Massachusetts;pa.i084,ny.i084,ct.i084,ma.i084
I-85;;al.i085,ga.i085,sc.i085,nc.i085,va.i085
I-86;Idaho;id.i086
I-86;Pennsylvania - New York;pa.i086,ny.i086,pa.i086swa,ny.i086bin
I-87;New York;ny.i087
I-87;North Carolina;nc.i087
I-88;Illinois;il.i088
I-88;New York;ny.i088
I-89;;nh.i089,vt.i089
I-90;;wa.i090,id.i090,mt.i090,wy.i090,sd.i090,mn.i090,wi.i090,il.i090,in.i090,oh.i090,pa.i090,ny.i090,ma.i090
I-91;;ct.i091,ma.i091,vt.i091
I-93;;ma.i093,nh.i093,vt.i093
I-94;;mt.i094,nd.i094,mn.i094,wi.i094,il.i094,in.i094,mi.i094
I-95;;fl.i095,ga.i095,sc.i095,nc.i095,va.i095,dc.i095,md.i095,de.i095,pa.i095,nj.i095,ny.i095,ct.i095,ri.i095,ma.i095,nh.i095,me.i095,nj.i095sec
I-96;;mi.i096
I-97;;md.i097
I-99;;pa.i099,ny.i099
Whether it's worth the effort is a different matter.
*Of course there's the business routes in another system, but that makes it more complex - I ditched the system column so I could integrate usaib, but decided against doing it as it would be relatively easy here, but could get messy with multi-state routes and such.