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The Pennsylvania State Route Numbering System
The Interstate Highway System and Renumbering

The Interstate highway system consists of a planned system of over 42,000 miles of superhighways, better known as freeways. These highways differ from earlier highways in that they provide access to other road only via interchanges (no at-grade intersections), consist of at least two lanes for each direction of travel, divide opposing traffic lanes with a median, and allow safe travel at high speeds (generally at least 55 mph). There are further standards, including minimum radii of curvature for curves, maximum grades, and more.

The Interstate highways are marked by characteristic red, white, and blue shields. The highways also have a well-established numbering system based off the US highway numbering system, as explained by the Federal Highway Administration.

The interstate highway system was devised in the 1950s and was significantly completed by the end of the 1970s. In 1961, the Pennsylvania Department of Highways renumbered state routes that already had the numbers as the Interstate highways.

State routes with numbers duplicating the interstate route numbers were renumbered according to the following rule:

North-south 7x routes: For example, for renumbering 70, and 78, the numbers 171, 173, 175, 177, or 179 might be chosen. The lowest available number 171 was given to 70, and the next available number 173 to 78. (If I-76 had been finalized with I-70 and I-78, then 76 would also have been renumbered according to the rule: 70 may have been changed to 171, 76 to 173, and 78 to 175.)

East-west 7x routes: Only 79 fits into this category. Possible numbers for 79 were 170, 172, 174, 176, and 178. 170 and 176 were discarded from consideration because they were x70 and x76 routes and might be used as future Interstate numbers, and 174 was discarded because it was already assigned. 172 would have been the obvious choice, but for whatever reason, 178 was chosen, even though this would have duplicated the proposed (and later canceled) I-178.

North-south 8x routes: 83 was split in two, and the northern segment (north of Reading) ran north-south. 84 was also a north-south route. The possible numbers for 83 and 84 were 181, 183, 185, 187, and 189. 187 was already assigned, and 181 was assigned at this time. 183, 185, and 189 were free numbers, and 183 was selected to replace 83 (without reserving 183 as a future interstate number). 84 should have been 185, but instead received a 200-series number. The possible 28x numbers were 281, 283, 285, 287, and 289. 281, 283, and 285 were already assigned, but 287 and 289 were free numbers. 287 was assigned to 84. 86 is also a north-south route, but I-86 did not appear until the late 1990s, well after the renumbering rule was applied.

East-west 8x routes: 80 and the southern part of 83 were east-west routes. 81 had also been an east-west route, but was replaced by US 40 decades before the Interstate system was devised. Possible numbers for 80 and 83 were 180, 182, 184, 186, and 188. 180 was already being renumbered to avoid duplication with I-180 (now I-176), and thus was discarded. 182 was assigned in York County at this time. 184 and 188 were already assigned. 186 was a free number, but may not have been chosen because a 186 once intersected 80. None of these 18x numbers were found to be appropriate. Possible 28x numbers were 280, 282, 284, 286, and 288. 280 was being renumbered like 180 to avoid duplication with I-280 (now I-276). 282, 284, and 288 were already assigned, and so most of 80 became 286. For whatever reason, the southern part of 83 was renumbered to 724.

North-south 9x routes: Only 90 fit into this category, and the lowest possible replacement number was 191, which was available, and so 90 became 191. I-99 was not assigned until the 1990s, well after the renumbering rule was effected, and in fact, 99 still exists.

East-west 9x routes: Only 95 fit into this category, and the possible replacement numbers were 190, 192, 194, 196, and 198. 190 was reserved as a future interstate number, and so 192 was chosen to replace 95.

Three-digit routes: See the table below.

The renumbering rule was well followed for avoiding duplication with the original interstate highways (like I-79, I-80, and I-83) but was mainly ignored for other Interstate highways (I-76, I-283) and especially for recent additions to the Interstate highway system (I-86, I-99, I-380)..

Renumbered State Routes

The following table summarizes the state routes that were renumbered to avoid route number duplications with interstate highways.

Interstate	Renumbered Route	Remarks
		Renumbered to 171 and runs north-south. The other x70 routes remain.
		Renumbered later (in 1964) to 655, not in accordance to the renumbering rule, but runs north-south.
		Renumbered to 475 and runs north-south.
		Renumbered to 747 and runs north-south.
		Renumbered to 829 and runs north-south.
		Decommissioned before the Interstates were assigned.
		Never assigned. I-576 is the future number for the Southern Beltway bypass of Pittsburgh.
		Never assigned.
		Never assigned.
		Renumbered to 173 and runs north-south.
		Replaced 79 and was later decommissioned. Ran east-west.
		Renumbered to 478 but runs north-south. I-378 later became 378.
		Renumbered to 178 and ran east-west.
		Renumbered to 226 and runs east-west.
		Never assigned.
		Never assigned.
		Never assigned.
		Renumbered to 286 and partially replaced by 380. Runs east-west.
		Renumbered to 130 and runs east-west.
		Renumbered to 791 and runs north-south.
		Internally renumbered as SR 400, but remains signed as PA 380. I-380 was a later addition (originally I-81E and I-81S).
		Replaced by 403 and runs north-south.
		Renumbered to 217 and runs north-south.
		Decommissioned long before I-81 entered the state.
		Renumbered to 183 (north-south) and 724 (east-west).
		The original 283 was decommissioned long before I-283. A newer 283 was signed north-south and was later replaced by I-283, and then yet another 283 (SR 300, east-west) was built connecting US 30 and I-283.
		Renumbered as 287 and runs north-south.
		Internally renumbered as SR 886, but still signed as PA 86. The SR renumbering could not follow the precedent of 283 (SR 300) and 380 (SR 400) because 100 is SR 100. (If 86 had been renumbered according to the old renumbering rule, 86 would have become 185.)
		Renumbered to 191 and runs north-south.
		Renumbered to 192 and runs east-west.
		May be renumbered when I-295 comes to Pennsylvania.
		Never assigned.
		Never renumbered.
		Never renumbered. (If 99 were renumbered according to the old renumbering rule, 99 would become 193. 99's SR number (SR 99) cannot be renumbered like like 283 and 380 because SR 100 is taken by PA 100. 99 cannot be renumbered like 86 (by annexing a hundreds digit that matches the tens digit) because SR 999 is already taken by PA 999.)

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