background retrieving from video

hi I want to retrieve background of a street from a video So i wrote this code

if true    
close all;
clc;
obj= mmreader('12.avi');
Frames = obj.NumberOfFrames;
Height = obj.Height;
Width = obj.Width;
size(obj);
a=read(obj);
sum = double(zeros(Height,Width,3));
imframe = double(zeros(Height,Width,3));
for f=1:Frames   
      sum(:,:,:,:) = sum(:,:,:,:) + double(a(:,:,:,f));
end
imframe = (sum(:,:,:,:)/Frames);
imframe1 = uint8(imframe);
figure;
imshow(imframe1);
end

the result as shown in below not very good

I want to delete the cars that are in specified area how I can do that please help me if any one know. best regards