While I suspect more could be said on the specific dynamics of trade in the era of the Arsacids, as I note in this post the entire popular conception of the 'Silk Road' is wrong. Throughout history, Chinese caravans rarely ventured beyond the Tarim Basin, let alone as far as Iran, not necessarily because they were physically or legally prevented from doing so but because making a greater number of shorter trips was more profitable and less risky than making a few long ones. Individuals and even some wares could and did travel great distances at times, but in general, the movement of goods and ideas across the Eurasian continent before direct maritime links were forged in the fifteenth and sixteenth centuries was an emergent property of a large number of local and regional exchanges, and almost never due to people making an entire journey from Europe to Asia or vice versa. That is not to say that there were never instances of such travels, but these were principally by diplomats, priests, and statesmen and women, as well as the occasional enterprising traveller.