need to extract road using dog filter but its not working properly..help me out

fontSize = 10;

I = imread('a8.jpg');

grayImage = rgb2gray(I);

% numberOfColorBands should be = 1.

[rows columns numberOfColorBands] = size(grayImage);

% Display the original gray scale image.

subplot(2,2,1);

imshow(I);

title('Original Image', 'FontSize', fontSize);

subplot(2,2,2);

imshow(grayImage);

title('grayscale Image', 'FontSize', fontSize);

gaussian1 = fspecial('Gaussian', 21, 15);

gaussian2 = fspecial('Gaussian', 21, 20);

dog = gaussian1 - gaussian2;

dogFilterImage = conv2(double(grayImage), dog, 'same');

subplot(2,2,3);

imshow(dogFilterImage, []);

title('DOG Filtered Image', 'FontSize', fontSize);

bw =(dogFilterImage);

subplot(2,2,4);

imshow(bw);