Any of you that live in big cities no that bridges often go through extremely costly upgrades. It is often cheaper and less disruptive than building a new bridge, which is why they upgrade them. It is also often rather half-assed, and no nearly sufficient. As well, there are often safety concerns
on the new bridge. Yes it is "safe" enough by some study, but it certainly isn't as safe as the original after they cram another lane in.
Now, in a perfect world, the bridges would just be built good enough in the first place. However, often what is needed isn't the only factor in what gets built. Other concerns, such as cost, public image, politics, etc. all come into play. Even though they know a bridge will only be sufficient for 25 years, it is hard to convince people that they should pay for something that they will never utilize when the hospitals need money too.
So in comes the "upgradeable bridge." Say they are looking at a 4-lane bridge. If they buy the upgradeable version, then it is more expensive than a regular 4-lane, by some percentage, however not too much, say 10-20%. The major load bearing pieces (supports, cables, etc.) are engineered to be able to hold much more than the 4 lane bridge.
The public sees this as a bridge that solves/helps there current traffic problems, and isn't unreasonably expensive.
Go down 25 years down the road, and traffic is again a problem. So at that point, you upgrade the bridge to the 6-lane version. You have to do very little alterations to what is already there, maybe change an on-ramp a little or move a divider. New support for the extra lanes attach in predetermined places to the original supports, and things go in, fitting properly and safely. The cost here may be something like 110-120% of the original difference between a 4 and 6 lane bridge.
So when it is all said and done, the city has bought a 6 lane bridge for ~130% of a standard ones cost. However, they have done it in distinct stages spending money at different times, appeasing the public. This results in less "one-time" money needed. The public is happy because they don't see a giant bridge that is not being used. When the upgrade is made, safety is not compromised because the original plan calls for the extra lanes.
Depending on the bridge, the extra lanes could be a second level, or on the outside of the current lanes. As well even if the city never upgrades the bridge, the company making still makes money on the original. This isn't something that little one horse towns use, it would be something for major cities where it is extremely unlikely that growth is going to stop, so there is definite appeal to knowing that a bridge can be upgraded in the future.