I'm not really sure what the worry is.
The A1 side/blackcat roundabout hotspot is being sorted, ground already broken to turn that into a flyover with A1 and continue the road along to Cambridge and thence to Ipswich. M1 side decent, have never really had long delays specific to turning off the M1 onto the A421 even at busy times (was driving that way today and noticed they've left space for an extra couple of lanes on the central reservation where the A421 meets the M1 too). The A421 dual carriageway is pretty wide with extra lanes for some of the bigger junctions and verges/hard shoulders.
I guess there's a question of where traffic will leave the A421 to access the site?
I used to come off the A421 at the A5141 junction (the one on the right in the map below) as I lived in that part of town and it was never bad even during rush hour. However, getting onto the B530 which goes down the east side of the Universal site is a bit of a pain - you have to come off the A421 at the A5141, go north to the B530 and turn left past interchange retail park - badly planned road there and lots of queues from people going to the retail park. I doubt that would be the way it would be accessed though.
I'd guess access would be from the left junction on the map though anyway (by Maypole Farm pub). Bigger junction, most traffic coming off it is heading North around the A6 bypass, and A421 goes to 3 lanes for a bit to keep traffic flowing on the East side. I'd guess the car park would be accessed from there by turning on to Manor Road. Probably some improvements could be made there but it's nothing earth shattering - pretty small scale road changes to keep things flowing. The area on the left of the map is absolutely filled with distribution centres. Massive new places with dozens of truck bays that have been going up over the past few years specifically because the road infrastructure is good.
@tommyhawkins - where did you reckon the car park would be again?
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