I have wrote a simulation in python based on Nagel–Schreckenberg model but It dont seems to work do you know why?

import numpy as np
import random
import time
from IPython.display import clear_output

def delay():
    time.sleep(1)

L = 100      #length
d = .22      #density
start_s = 1  #start speed
max_s = 5    #max speed
acc = 1      #acceleration
rand = 0.1 

#initialise
road = np.zeros(L, dtype='int32')
for i in range(road.size):
    if random.random() < d:
        road[i] = start_s

def mod(x):
    return x % L

#simulation
while True:
    clear_output()
    print(road)

    for i in range(L):

        if not road[i] == 0: #veh

            if road[i] < max_s: #acc
                road[i] += acc

            for j in range(max_s): #slow
                if not road[mod(i + j +1)] == 0:
                    road[i] = j + 1
                    break

            if (random.random() < rand) and road[i] > 1: #rand
                road[i] -= 1

    new_road = np.zeros(L, dtype='int32')
    for i in range(L):
        if not road[i] == 0:
            new_road[mod(i+road[i])] = road[i]
    road = new_road

    delay()

So why do it prints something like this
[0 0 0 4 0 0 0 4 1 0 2 0 0 0 0 0 5 0 0 0 4 0 2 0 0 0 4 0 2 0 0 0 0 4 0 0 0
0 5 0 0 0 4 0 0 0 0 5 0 0 0 0 5 0 0 0 0 4 0 0 0 0 4 0 0 3 0 0 0 4 0 0 0 0
0 0 3 0 0 0 4 0 0 0 0 0 4 1 0 2 0 2 0 0 3 0 0 0 0 5]
and freezes?

I tried making a new numpy array or maby array of arrays