The central area problem is certainly an issue for the airport right now - traffic is frequently backed up to the Ibis roundabout. I fear that once the new dual carriageway is in place the situation may become even worse, since it will enable traffic to reach the central area at an even faster rate.
The frustrating thing is that the employment of a single traffic steward, plus letting taxis back into the 2 rows next to the terminal which remain blocked off, would do so much to improve the congestion.
Interesting news about the increase in passengers using Luton Airport Parkway - although the number of non-airport related London commuters (like myself) who use the station is unknown. This however is another problem area, with the shuttle busses often full to bursting at peak times. I often see a full bus leave with still dozens of people waiting at the station unable to board - some of whom resort to paying for taxis. The airport needs to realise that at certain times of the day the shuttle bus frequency probably needs to be doubled.