do you mean for this to be something permanent that happens when images are being processed or something that you would switch on/off when viewing the map? the latter sounds like a better option since you could turn it on when viewing somewhere busy and turn it off for all the other places that dont need it.
as well as just snapping to the road, it would be good to have them grouped on either side of the road depending on which direction they are going. (although you would need to have different rules depending on which country and side of the road you are driving on)
another thing that might help clean up things a bit would be to group together images that were taken at same time, i.e. if someone is using 4 cameras but they are all using separate gps which would mean a lot of messy lines
a way around the problem of sidewalk images being snapped to the road might be a slider to choose how aggressive the snapping would be or how much you want them smoothed out
anyway, personally i havnt had too much of a problem with messy sequences since im mostly using mapillary through the osm editor. i find its a bit easier to see at a glance whats going on since it shows the direction of each image without having to hover over each one like you have to in the main mapillary site…
so maybe a lot of confusion could be avoided by having an option to show the image direction for all images? it would also be a much quicker solution than having to program all of the above features and calculate which roads a sequence should be snapped to and all that