for this bridge he gives the following information:
Name: Bridge of Alcántara
Location: Alcántara
Country: Spanien
Construction type: Arch bridge
Completion date: 100 AD.
Builder: Gajus Julius Lacer
Current traffic use: Car
Material: Sandstone
Overall length:180 M
Bridge span:30 M
Height over water:50 M

This reminds me of a childrens game were you had different cars, or ships and you would name one quality, Top speed and if the other person had a slower car, you would get his card.

Do visit his site Herr Nebel has some fantastic pictures on there.

Here is his complete description:
Nearly 2000 years ago in the only thinly settled Extremadura a bridge was being built that remains the best conserved bridge build by the Romans. It is in the Alcántara in the Spanish/Portuguese border area where the Rio Tajo cuts a deep ravine into the peculiar landscape of the Extremadura. The Romans called the river "Tagus Aurifer" and the Portuguese was called calls it "Tejo".  In Lisbon it finally flows into the Atlantic.

The Roman emperor Trajan ordered the building of a bridge over the Tajo in approx. 97 A.D., in order to connect the city Cáceres with the region Beira Alta in today's Portugal.  

Today this bridge is considered as the architecturally most beautiful and best preserved Roman stone arch bridge and gives us a descriptive example of the amazing abilities of the Romans in the area of the bridge construction.  Such an imposing structure, far from the center of power of the Roman Imperiums, did not surely only have economic and strategic meaning for its designers.  The Roman realm reached during the reign Trajans its largest expansion and the bridge was certainly also a powerful demonstration of the technical and military superiority of Roman power.  For the execution of the construction work the building master Gaius Julius Lacer was assigned.  The financing was left to the 11 surrounding cities in the province Lusitania.  
The construction work lasted from 98 to 103 A.D. and was mainly build by.  Even if this bridge does not rank among the largest stone arch bridges build by the Romans, it is because of its height and its architectural harmony a masterpiece.  The roadway is approx..  50 meters over the normal water level of the Tajo.  The two centre arches have spans of nearly 30 meters and to there are the furthest arches build in the antique.  In the center of the bridge is the triumpharch for emperor Trajan, who has a height of approximately 14 meters.  

In the year 1543 emperor Karl V. arranged an extensive restoration of the bridge and added double eagle coat of arms at the triumph arch of the Trajan fastens.  

The bridge of Alcántara defies nevertheless for nearly 2000 years the tides of the Rio Tajo and today more than ever by a substantial volume of traffic is taken up.  An amazing stability, if one considers that the stones are set on each other without any mortar!  Nothing much is known about the building master Gaius Julius Lacer except his participation in the building of the bridge of Alcantara.  He must have been very probably conscious of the fact that something great was created here: a small temple directly next to the bridge was build to honour him and where he was.  An inscription on his grave plate reads:  "I leave this bridge for all times to the generations of the world".  The Romans built for eternity...
and as always please post your own favourite bridges.