I think they may need the roundabout info for more than one reason. They obviously have other info that we don't know about so they are trying to put the pieces together, so yes, to test an alibi, maybe to also match up with mobile phone records and the timing of everything, and possibly also to identify something out of the ordinary at the roundabout.
In addition to some of the things others have suggested which could be classified as out of the ordinary, I have also wondered if Allison wasn't killed at home. If there was a fight and she took off and he chased after her, or he went outside to get into the car - and she ran after him and got into the car and they continued arguing as he drove. Just some sort of frenzied situation which resulted in him stopping the car somewhere and he killed her in the car after the argument escalated. This might seem unlikely, but I have been in a situation where an ex of mine who had an anger problem would throw some horrific insult combined with some obscure piece of information designed to upset me,or a blatant lie or twisting of the truth about me which was going to inflame me or make me feel I needed to explain myself, then storm out before I had a chance to respond or defend myself. It was like a very dramatic tactic of having the last word in an unfair verbal attack that was going to worry me and hurt me deeply. Most of the time I just let it go but a couple of times I became so frustrated and sick of feeling disempowered I ran after him and tried to get into the car. On one occasion I got into the car and he took off in a temper driving really fast to scare and intimidate me.