Look at these, I use them all the time, the speed is how fast the traffic goes over them and use <kuid2:95230:101399:1> Jons Bridge Support Stone if you need more support:
<kuid:95230:102612> Jons Bridge L6 DL3 BR-Si+Sh,65mph NT
<kuid2:95230:100652:3> Jons Bridge L2 DL2-Si+Sh,45mph NT...