About 2,700 competitors took part in what organisers called the UK's "biggest-ever" Ironman event in Weymouth in September.
Dorset County Council said it would investigate "delays and confusion" on the roads following complaints from motorists.
Organisers said next year's event will not have a 140.6 mile (226.3km) option.
Drivers on the day reported taking three hours to make a four-mile journey between Weymouth and Dorchester.
The competition involves a swim in Weymouth bay followed by a run through Weymouth and a cycle through rural west Dorset.
September's event included both a 70.3 mile (113.2km) and the full distance 140.6 mile (226.3km) route.
Dorset Police said traffic disruption on the day of the event was made worse by vandals who removed road signs.
Dorset County Council's audit and governance committee chairman Trevor Jones said:  "Unfortunately, this is not the first time that events such as these have experienced problems. This is despite previous assurances by organisers that issues would not be repeated.
"We have therefore decided to investigate the circumstances so we can scrutinise and make recommendations for necessary improvements."
Speaking following the decision to only use a 70.3 mile (113.2km) route in 2017, Ironman UK chief executive Kevin Stewart said: "We know the important part is to see the economic impact with more visitors to the area - there is a focus there to make sure it is a big and successful race going forward."
An inquiry is being launched following traffic disruption during a triathlon event.