Like in FH3 there are glitched streets.... again!
the achievement will pop up at 529/531 so don´t worry. (this is still the case by December 2019)
Focus on the red roads if you go for the grind. ALWAYS check it they are counted. if nothing happens on the counter, drive them again.
Do it as soon as you can drive in the open world. dont mess around, drive ON the streets and roads and have an eye on the street counter. drive the red roads ALWAYS till the end. sometimes the map shows a road done and cleared but it doesnt count because the car didnt finished the road.
if you are confused with this information, here is an example:this line is the road in the game:
------X------------X----------X---------X------C
at every X the street color marking on the map will change BUT you must reach C to activate the counter.
In this example you see 4 X. If you reach the 4th X the street maybe appear complete on the map but the street wont count at this point. This is why you need to drive until the counter would go from 0/531 to 1/531
every street is different here. sometimes they count early.
the hoonigan focus is a good car to make it. it can handle streets and off-road in the same way. i made a tuning setup for this car called XP grind.
upgrade the perks of this car asap and you will see what will happen while just driving around

you can drive this car on every ground in every season.
make sure that you drive trough all TUNNELS and jump all RAMPS in area like mudkickers adventure park and mortimer gardens. dont zig zag around in that areas. show discipline while unlocking the streets there.
Again:-Start as soon as possible (dont race around or you make it harder)
-ALWAYS drive on the roads & avoid jumping
-if a road ends, keep driving UNTIL THE COUNTER GOT THE ROAD
-dont rely on the markings on the minimap/map, rely on the counter
-still have on eye on the minimap to check if the streets change their color.
photos of some special spots:
https://imgur.com/a/ni6LwCDhttps://imgur.com/a/9jYaw4Uhttps://imgur.com/a/QV6AX9Ohttps://imgur.com/a/61zTVmy