examples/open_world_city/Assets/Scripts/RoadNetwork.cs

using System.Collections.Generic;
using UnityEngine;

namespace UnityAgent
{
    /// <summary>
    /// Lays out a grid road network at runtime: builds thin box meshes for
    /// roads and an intersection marker at every grid crossing.
    /// </summary>
    public class RoadNetwork : MonoBehaviour
    {
        [Header("Grid")]
        public int gridSize = 5;
        public float blockSpacing = 30f;
        public float roadWidth = 6f;
        public float roadThickness = 0.1f;

        [Header("Materials")]
        public Material roadMaterial;
        public Material intersectionMaterial;

        private readonly List<GameObject> _roads = new List<GameObject>();

        private void Start()
        {
            Build();
        }

        public void Build()
        {
            foreach (var r in _roads) if (r != null) Destroy(r);
            _roads.Clear();

            float total = (gridSize - 1) * blockSpacing;
            Vector3 origin = transform.position - new Vector3(total * 0.5f, 0f, total * 0.5f);

            for (int i = 0; i < gridSize; i++)
            {
                // Horizontal road
                _roads.Add(SpawnRoad(
                    new Vector3(origin.x + total * 0.5f, 0f, origin.z + i * blockSpacing),
                    new Vector3(total + roadWidth, roadThickness, roadWidth),
                    roadMaterial, $"Road_H_{i}"));

                // Vertical road
                _roads.Add(SpawnRoad(
                    new Vector3(origin.x + i * blockSpacing, 0f, origin.z + total * 0.5f),
                    new Vector3(roadWidth, roadThickness, total + roadWidth),
                    roadMaterial, $"Road_V_{i}"));
            }

            // Intersections.
            for (int x = 0; x < gridSize; x++)
            {
                for (int z = 0; z < gridSize; z++)
                {
                    Vector3 p = new Vector3(origin.x + x * blockSpacing, 0f, origin.z + z * blockSpacing);
                    _roads.Add(SpawnRoad(p, new Vector3(roadWidth * 1.2f, roadThickness * 1.5f, roadWidth * 1.2f),
                                          intersectionMaterial, $"Intersection_{x}_{z}"));
                }
            }
        }

        private GameObject SpawnRoad(Vector3 pos, Vector3 scale, Material mat, string name)
        {
            GameObject go = GameObject.CreatePrimitive(PrimitiveType.Cube);
            go.name = name;
            go.transform.SetParent(transform, false);
            go.transform.position = pos + transform.position;
            go.transform.localScale = scale;
            if (mat != null) go.GetComponent<MeshRenderer>().sharedMaterial = mat;
            return go;
        }
    }
}