A quick way to instantly improve car accessibility/routing calculations is to introduce historic speed data.  This will provide accurate speeds primarily taken from GPS devices attached to the car, along with other forms which vary by supplier.  This data is then appended to the link and (differs per supplier) grouped by individual times of a day or a summary of peak, off peak, night time etc.   

With software packages like Accession, you can utilise premium speed datasets to provide much more accurate drive-time speed catchment isochrones.  In a Basemap pilot study the difference between the DfT recommended speeds for an A road and some ITIS Speed data were 60m/h different.  These changes can make a massive difference when trying to produce travel time isochrones.

If you would like some additional information on speed data or would like to order some of this data then please contact us.