Neil,
Alltec do a couple of traffic lane cleaners as do Prochem, Extracta Etc.
It is necessary to allow dwell time for the surfactants to break down the dirt.
This can be accelerated by agitation , i.e agitating with a stiff brush.
Alternatively you can use microsplitters which do not require dwell time but need agitaion.
I suggest you try a few different ones and see which works best for you.
I have Alltec,Prochem , Craftex Ms and Solutions MS on my van plus a couple of my own formulations.
Cheers,
Doug