Python 无法显示其线条的表面

我正试着在一个表面上画一条线，然后我在显示器上涂鸦. 当我这样做时，只有表面出现，而线没有绘制.

我写的代码是

import pygame as pg
import numpy as np

pg.init()

# Define constants for the screen width and height
SCREEN_WIDTH = 750
SCREEN_HEIGHT = 500

# Create the screen object
screen = pg.display.set_mode((SCREEN_WIDTH, SCREEN_HEIGHT))
screen.fill((40,158,80))

running = True
while running:
    for event in pg.event.get():
        if event.type == pg.QUIT:
            running = False    

    # Road surface
    road_height = 100
    road = pg.Surface((SCREEN_WIDTH+10,road_height))
    road.fill((0,0,0))
    road_topleft = [0,(SCREEN_HEIGHT-road_height)/2]
    
    # Side lines
    st_ratio = 0.0375 # side to total road width ratio
    sl_width = int(np.rint(st_ratio*road_height))
    sideline_topleft = road_topleft+[0,st_ratio*SCREEN_HEIGHT]

    pg.draw.line(road, (255,255,255),[0, (SCREEN_HEIGHT-road_height)/2+st_ratio*road_height], \
                 [SCREEN_WIDTH, (SCREEN_HEIGHT-road_height)/2+st_ratio*road_height], sl_width)
    
    screen.blit(road, road_topleft)
    pg.display.flip()

pg.quit()

我的 idea 是设计一条道路，我目前正试图将线条添加到"道路"表面. 然而，我得到的输出是

Output screen个