Sometimes you don't have an ND filter with you or one that will fit the lens you have on you, or like me I really wanted to use my 7mm wide angle lens on my m4/3 Olympus EM-5 body but I have no ND filter to fit the larger lens.
There is there another way to shoot a long exposure but it does require a little more work in post!
The idea is to take multiple short long exposures (yes there is such a thing :)) and using the power of Photoshop blending them together to achieve the desired result. This method can be used in the middle of the day but would require a whole lot of images. iPhones have the ability to turn a Live Photo into an long exposure doing it this same way, something I wish I could do on my Pixel 2!
To do this you need to push the length of the long exposure as much as you can, for me with the photo above of the Sacred Heart Cathedral it was 8 seconds per exposure and I waited for more cars to drive past for each exposure. It's best to use a Remote or Shutter Cable Release or a timer to avoid any camera movement between shots - this pays off big time later on!.
Once you have a few photos you are happy with it's time to do a little magic and create that long exposure you could see in you head.
I use Adobe Lightroom and Photoshop CC. After importing into Lightroom I reviewed the exposures that would be making up the final image and edited one of them, synced the changes across all then opened all images in Photoshop as Layers - this is done by right clicking on one of the selected images and selecting 'Edit in' then 'Open as Layers in Photoshop'.
In Photoshop you select all layers but the bottom one then choose 'Lighten' from the layer blending options and you have the light from each exposure overlaid over the bottom exposure giving that very long exposure look - this is also how you can do star trails!
I did some extra work on the clouds for this one because the look I got was not smooth due to the time difference between exposures waiting for cars, this is done a slightly different way but that is for a different time.
Have a look at the video below that is a time-lapse of how I created this image.