Interesting question. If you approach in a southerly direction via Swanland Road or St Albans Rd then you are on a 60 m.p.h. single carriageway so it is difficult to see how it could become 70 m.p.h. West of the BP filling station. On the other hand if you leave the A1(M) Southbound and enter the roundabout then the limit on the roundabout is 70 according to a local police officer