Unity

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class Road : MonoBehaviour {
    UnityEngine.AI.NavMeshAgent playerAgent;
    float speed = 15f;
    private bool onRoad = false;

    void Update () {
        if (onRoad) {
            playerAgent = GetComponent<UnityEngine.AI.NavMeshAgent> ();
            playerAgent.speed = speed;
        }
    }

    void OnTriggerEnter(Collider other){
        if (other.gameObject.CompareTag("Road")) {
            onRoad = true;
        }
    }

    void OnTriggerExit(Collider other){
        if (other.gameObject.CompareTag("Road")) {
            onRoad = false;
        }
    }
}