They're also strong and relatively light, which is a consideration for drawbridges.
To answer the OP's question, I'm okay riding over these bridges on 25mm road tires. I have Gatorskins on my training wheels and don't worry about them. On the other hand, when it rains those things are slick as snot and you have to have a very smooth pedaling action and very light touch on the bars. Or I'll walk--no shame in that.