I'd be a bit worried if my car ran hotter in traffic than on the motorway as it would either indicate that it's overheating in traffic or not at operating temperature on the motorway. Or do you just mean it warms up quicker in traffic?
Incidentally 8 miles won't necessarily bring a car up to operating temperature, my Ford S-Max 1.8 diesel (luckily no DPF
) takes the first 10 miles of my 30 mile commute to warm up fully at this time of year.
Depending on the setup of the vehicle DPF regeneration heats the exhaust to a temperature of 400°C-600°C or higher. This level of heat may become noticable to the driver in statonery or slow moving traffic.