Wood vs Steel Deflection Criteria 1

The American Institute of Timber Construction (AITC) recommends live load deflection criteria of L/300 for timber bridges. AASHTO recommends L/500; other criteria are stated to be acceptable. Considering traffic volume, speed limits, relative deflections of components in the bridge structure, and field experience, a deflection limit of L/300 is generally sufficient for timber bridges.
The logic behind this approach is primary very low Young Modulus for timber - so deflection for a timber bridge is usually large. Limiting the deflection to L/500 will lead to oversized members, as the displacements will govern the design.
Also, due to the excellent vibration-damping characteristic of the timber structures, larger deflection will be not so uncomfortable to the users.
It should be added that the deflection criteria is mostly a "comfort" requirement in the design process, and large deflection does not necessarily mean that the structure is unsafe.