A new bridge at Ashton Gate for the MetroBus will have to be increased in height - at a cost of at least £1million a metre.
The bus-only bridge has had to be increased in height in case the rail track between Portishead and Temple Meads is eventually electrified after the line starts taking passenger trains.
Preparatory work on the "Skew Bridge", which will take the new single deckers over the railway line which runs parallel to Winterstoke Road, has already begun.
When the bridge was originally designed, there was no talk about the prospects of the Portishead line being electrified.
But when the MetroBus designers realised that electrification was a possibility - albeit some years in the future - theytook the decision to redraw the plans so it could accommodate the overhead gantries for the electric cables.
The revised bridge is longer and two metres higher - and the cost has increased from £6million to between £8-9million.
Facts and figures:
+ Original cost of the bridge: expected to be around £6m
+ Cost of redesigned bridge: £8-9m
+ The overall length of the bridge and approach ramps is approximately 340m.
+ The reopened Portishead line will increase the Britain's passenger rail network by 10 miles and connect an extra 35,000 people to the railway network.
+ MetroBus is expected to carry more than 20,000 passengers each day more than double the 7,500 passengers who daily use the Severn Beach railway line and more than 15 per cent of the 128,000 passengers who use First Group's buses in Bristol on an average weekday.
+ The number of passengers who will use MetroBus instead of travelling by car is forecast to be 2,000 per weekday and 600,000 per year.
These passenger numbers are for the first year only and would be expected to grow.
Estimates of initial passenger numbers often prove to be conservative, when the Cambridge Busway opened passenger numbers were 46 per cent higher than predicted.
The re-design has not altered the target date for this MetroBus route coming into use by the summer next year because other works - such as the new bridge at Bathurst Basin next to the Louisiana pub - will take longer.
Building work will start within the next couple of weeks and lead to road diversions in the area for several months.
A spokesperson for MetroBus said: "The bridge has been designed to allow MetroBus to cross the railway line without taking space from the road.
"The change in height is to ensure that the forthcoming Portishead railway line can be electrified in the future.
"The delay caused by the changes to the bridge will not impact on the start date of MetroBus services."
The spokesperson explained that the new Skew Bridge would mean MetroBuses will be able to avoid the busy Cumberland Basin which usually sees traffic grind to a halt at peak times.
Which roadworks will be in place while Skew Bridge is being built?
A slip road from Winterstoke Road to the Cumberland Basin will be closed to traffic while the Skew Bridge is under construction.
There will also be temporary traffic lights at the level crossing which gives access to the industrial estate off Winterstoke Road.
Construction will start with the piling of the bridge foundations, followed by building the approach ramps and the high bridge piers and abutments.
The bridge's beams will be lifted into place in autumn/winter 2016 which will be followed by the final construction of the bridge's deck.
Access to Ashton Vale Road industrial estate will be maintained during the construction works.
When the bridge is completed, the traffic light system at the level crossing junction will be upgraded.
