Simplify the following text for a Grade 4 native English-speaking person.
Original: Tickford Bridge , over the River Ouzel ( or Lovat ) , was built in 1810 and is the only iron bridge in Britain that still carries main road traffic and is the oldest iron bridge in the world that is still in constant use .
Simplified: It is the only iron bridge in Britain that still carries main road traffic . This is the oldest iron bridge in the world that is still in constant use .