rand() in C with modulo never triggering

I'm trying to implement a real-world simulation involving synchronization, and when I have an event that has an 80% chance of occurring, I'm currently doing

  while((rand()%10)<8){
    up(sCar);
    printf("SOUTH: new car\n");
  }

However, the while loop is never triggering while ran, so I'm not sure if I'm using rand() properly. If I replace the rand() with 7, when it works properly. I currently set

  srand (time(NULL));

earlier in my program as well. Any help would be much appreciated.

EDIT: Here is the full running program. I have modified sys.c to create the system calls for up and down, which act as Semaphores.

#include <linux/unistd.h>
#include <stdio.h>
#include <sys/mman.h>
#include <stdlib.h>
#include <time.h>

struct cs1550_sem{
    int value;
    struct listnode *start;
    struct listnode *finish;
};

void up(struct cs1550_sem *sem) {
  syscall(__NR_cs1550_up, sem);
}

void down(struct cs1550_sem *sem) {
  syscall(__NR_cs1550_down, sem);
}

int main(void){

  srand (time(NULL));
  void * ptr = mmap(NULL, sizeof(struct cs1550_sem)*3, PROT_READ|PROT_WRITE, MAP_SHARED|MAP_ANONYMOUS, 0, 0);

  struct cs1550_sem *nCar = ((struct cs1550_sem *)ptr);
  struct cs1550_sem *sCar = ((struct cs1550_sem *)ptr) + 1;
  struct cs1550_sem *mutex = ((struct cs1550_sem *)ptr) + 2;
  struct cs1550_sem *flag = ((struct cs1550_sem *)ptr) + 3;

  void * northRoad = mmap(NULL, sizeof(int)*(10+1), PROT_READ|PROT_WRITE, MAP_SHARED|MAP_ANONYMOUS, 0, 0);
  void * southRoad = mmap(NULL, sizeof(int)*(10+1), PROT_READ|PROT_WRITE, MAP_SHARED|MAP_ANONYMOUS, 0, 0);

  nCar->value  = 0;
  nCar->start= NULL;
  nCar->finish    = NULL;

  sCar->value  = 0;
  sCar->start= NULL;
  sCar->finish  = NULL;

  flag->value  = 0;
  flag->start= NULL;
  flag->finish    = NULL;

  mutex->value  = 1;
  mutex->start= NULL;
  mutex->finish  = NULL;


  if(fork()==0){
    while(1){
      while((rand()%10)<8){
        up(nCar);
        printf("NORTH: new car\n");
      }
      printf("NORTH: no more cars, sleeping for 20 seconds\n");
      sleep(20);
    }
  }
  else if(fork()==0){
    while(1){
      while((rand()%10)<8){
        up(sCar);
        printf("SOUTH: new car\n");
      }
      printf("SOUTH: no more cars, sleeping for 20 seconds\n");
      sleep(20);
    }
  }
  else if(fork()==0){ 
    while(1){
      down(nCar);
      down(mutex); 
      printf("NORTH car allowed through\n");
      up(mutex);
    }
  }
  else{
    while(1){
      down(sCar);
      down(mutex);
      printf("SOUTH car allowed through\n");
      up(mutex);
    }
  }
  return 0;
}