UV offsets and splat maps issue

I am trying to add road placement functionality to my game. I've decided to go with a splatmap approach, where the splat map defines the road locations, and the road types. I use the following shader to accomplish this:

 Shader "Custom/Terrain" {
     Properties {
         _Color ("Color", Color) = (1,1,1,1)
         _MainTex ("Albedo (RGB)", 2D) = "white" {}
         _Glossiness ("Smoothness", Range(0,1)) = 0.5
         _Metallic ("Metallic", Range(0,1)) = 0.0
 
         // For roads
         _RoadMap ("Road Splat Map", 2D) = "white" 
         _RoadMapIDDirt ("Dirt Road Value", Float) = 128
         _RoadDirt ("Road Dirt Texture", 2D) = "white" {}
     }
     SubShader {
         Tags { "RenderType"="Opaque" }
         LOD 200
         
         CGPROGRAM
         // Physically based Standard lighting model, and enable shadows on all light types
         #pragma surface surf Standard fullforwardshadows
 
         // Use shader model 3.0 target, to get nicer looking lighting
         #pragma target 3.0
 
         // for debugging
         #pragma enable_d3d11_debug_symbols
 
         sampler2D _MainTex;
 
         // road stuff
         sampler2D _RoadMap;
         float _RoadMapIDDirt;
         sampler2D _RoadDirt;
 
         struct Input {
             float2 uv_MainTex;
             float2 uv_RoadMap;
         };
 
         half _Glossiness;
         half _Metallic;
         fixed4 _Color;
 
         void surf (Input IN, inout SurfaceOutputStandard o) {        
             // Albedo comes from a texture tinted by color
             fixed4 c = tex2D (_MainTex, IN.uv_MainTex) * _Color;
             o.Albedo = c.rgb;
             // Metallic and smoothness come from slider variables
             o.Metallic = _Metallic;
             o.Smoothness = _Glossiness;
             o.Alpha = c.a;
             
             // Handle roads
             // Roads are drawn using a splat map where one pixel is equivalent
             // to one tile in the terrain grid.
             // Color defines the road type
             fixed4 road = tex2D (_RoadMap, IN.uv_RoadMap - (0.5 / 128));
             if(road.a == 1) {
                 if(round(road.r * 255) == _RoadMapIDDirt) {
                     o.Albedo = tex2D (_RoadDirt, IN.uv_MainTex);                    
                 }
             }
         }
         ENDCG
     } 
     FallBack "Diffuse"
 }

The game is grid based, and currently is 128x128 tiles, defined by a 129x129 vertex plane. _RoadMap is a 128x128 pixel texture, with filter mode point.

The problem I am having is that all of the roads are offset by 0.5 grid units, and as such they show up between tiles, not on tiles. I understand this has to do with the way UVs are sampled, and I tried to fix this (as demonstrated in the above shader). While this fixes it in the dead center of the terrain, there is still a slight offset that increases the further away from center I get:

I use the following splat map:

How could I go about getting the road to line up with the grid properly?