The HSR is an overarching rating system that takes a look at all known factors that might make a bridge more or less historic and rates them on a scale from 1-10. The 0 rating is only used for bridges that are totally non-historic, generally, bridges newer than 1970. The HSR mathematically provides 121 combinations, although it is expected that a few of these combinations will not be used. The rating system does NOT indicate whether a bridge is eligible for listing in government programs such as the United States National Register of Historic Places, as government agencies have very specific factors they use to determine whether a bridge is historic, and many bridges that the general public might see historic value in might be dismissed as ineligible by the government. HistoricBridges.org has a more broad definition of historic, based more on what the general public and historic bridge enthusiasts consider to be of historic value.
Please also note that the scale and rating of bridges is evolving, particularly in locations where HistoricBridges.org's coverage is increasing as our understanding of bridges and their rarity in comparison with other bridges locally and nationally improves.
The national ratings portion of the scale is usually consistently implemented from bridge to bridge, because each bridge is being compared to the same set of bridges in the same area. Thus, it is possible to provide the below rough set of guidelines that explain what types of bridges might get a particular rating. Note that these are not exact delineators, but instead represent what types of structure are most likely to be placed at a particular level. Note that the examples are just that; they are not rules nor are they descriptions of bridges that receive that rating. Also, note that this table is based on mainly the technological significance of a bridge. If a bridge is historic due to association with a significant person or event in history, the rating may be higher than seen here. Under this system, very few bridges on the website will have a rating above 8 and below 2.
| HSR | Description | Examples |
| 0 | Bridges with no historic value whatsoever. | Bridges built after 1970. |
| 1 | Bridges barely old enough to be considered historic. Generally included because they retain remarkable historic integrity or are the last in a region. | 1960s pre-stressed concrete or steel stringer bridges. Small 1940s-1950s steel stringer bridges. |
| 2 | Bridges that are common technology, but may retain historic integrity or be large examples of their type, and thus are good examples of period technology. | Larger multi-span 1940s-1950s stringers. Old expressway overpasses. Small pre-1940 stringer bridges. Small concrete slab design bridges. Small rigid-frame bridges. |
| 3 | Relatively young bridges that are nevertheless an uncommon structure type, or otherwise have enough significance to be higher than 2. | Average to large size pre-1940 stringer bridges. Average plate girders. Outstanding expressway overpasses with excellent historic integrity and uncommon design. Average concrete t-beams. Concrete arch bridges with below-average historic integrity. Exceptional rigid-frame bridges. |
| 4 | These bridges are noteworthy, but may still be relatively common. Bridges here are generally at high-risk, and this rating should be balanced with the idea that bridges in this area of the scale are generally common, but due to fairly old designs, are also rapidly being demolished. | Large plate girders with good historic integrity. Extremely large, old, or unusual steel stringer bridges. Concrete girders. Exceptional concrete t-beams. Small riveted truss bridges with no unusual details or early construction date. |
| 5 | These are bridges are essentially the most common of the rare bridge types. Standard and traditional bridge designs are prevalent at this level. Bridges in this category clearly have historic value, yet are often facing demolition, and being assessed as non-historic. | Traditional riveted truss bridges with average or below integrity, and small pin-connected truss bridges with no unusual details or early construction date. Movable plate girders. Concrete arch bridges with good historic integrity. |
| 6 | Again, standard and traditional designs fill the majority of this category, but the bridges in this category are clearly rare and have significant historic value. Bridges in this category clearly have historic value, yet are often facing demolition, and being assessed as non-historic. | Riveted truss bridges with above-average historic integrity, later pin-connected truss bridges, and pin-connected truss bridges of any date with poor historic integrity. Post 1900 stone arch bridges. Larger concrete arch bridges with excellent historic integrity. Pre-1950 cantilever truss bridges. |
| 7 | These bridges are very important bridges and may predate standardization, or exhibit remarkable design. Generally, bridges at this level will be recognized as historic by the government, but may not be preserved. | Riveted truss bridges with unusual truss configuration (incl. double warren). Older pin-connected truss bridges, and pin-connected truss bridges with remarkable historic integrity. Pre-1900 stone arch bridges. Average concrete curved chord through girders. Movable truss bridges. Pre-1930 cantilever truss bridges. Suspension bridges. |
| 8 | Extremely rare truss bridges fall into this category, as well as uncommon truss bridges with remarkable historic integrity, or extremely old examples of their type. Bridges with unique design characteristics, or high levels of ornamentation. Generally, bridges at this level will be recognized as historic by the government, but may not be preserved. | Large, multi-span pin-connected truss bridges, older pin-connected highway Pennsylvania trusses, Whipple trusses, pin-connected Pratt truss bridges that are pre-1880. Large concrete curved chord through girder bridges with excellent historic integrity. Pre-1850 stone arch bridges. Highway truss swing bridges. Older suspension bridges. |
| 9 | Bridges that are extremely old and rare, and/or also feature extremely rare components fall into this category. With a few exceptions, governments have been recognizing the significance of structures at this level and preserving them. | Cast iron bridges. Pre-1800 stone arch bridges with average historic integrity. 1870s Bowstring truss bridges, lenticular truss bridges, and bridges with Phoenix or Keystone columns. Multi-span concrete curved chord through girder bridges. Movable bridges with first-of-kind technology, or extremely rare type. |
| 10 | At the national side of the scale, these are are the quintessential landmark bridges of the country as well as the bridges that are without a doubt the very last of their kind. Bridges in this category are generally all cared for and their wellbeing is often important to the general public. | Brooklyn Bridge, Bollman Truss Bridge, Zoarville Station Bridge, Triple Whipple Bridge, Golden Gate Bridge |
The local scale is less consistent because, as its name suggests, it is based on the unique surrounding environment of the county and state/province it is located in. For this reason, no one set of examples can be provided here. It is based on the idea that a historic bridges that might be common on a national scale might be the only remaining example of its kind in a certain area. For example, if Michigan only had one 1930s massive-membered truss bridge, this bridge might have a 9 local significance rating, while in Pennsylvania they might have a rating of 5, where this bridge is much more common.
On the local side of the scale, only bridges that are the absolute best in a local or regional area (such as a county or state) can achieve a 10 rating.
Since a great deal of the problems of historic bridges revolve around the way they are interpreted by the public and their government owners, a special clarification is needed. The government has set specific standards for what makes something "historic." This set of rules and regulations helps define a bridge's historic significance and also determine if it is eligible for recognition in the National Register of Historic Places, or is eligible for any preservation funding. However, this system has several problems for historic bridges, and to apply it to the narratives and discussions presented in this website would defeat the spirit of this website.
The fact is, many bridges that have historic value and deserve to be preserved are being considered non-historic under the current government system. The system does recognize the significance and decreasing population of certain historic bridge types, particularly metal truss bridges. It also is far too black and white. Either a bridge is eligible for the National Register of Historic Places, is actually on the National Register of Historic Places, or is non-historic. Many non-eligible bridges are still of value to the communities they serve, or offer us information about the bridges of the past.
Any bridge that is older than 1970 likely has some design characteristics that are not seen on modern bridges today, even if the only difference is the design of the guardrail. Thus, a bridge older than 1970, perhaps one built in 1965, might be a record and example of bridges in a certain period of time. Thus, the bridge has importance in relating to people what this period of time was like, in bridge construction, and perhaps other areas of life. However at the same time, it makes sense to observe that an 1870 bridge that is the last of its kind in the nation, likely deserves more attention and preservation funding than the 1965 bridge, which is likely to be one of many remaining similar structures. Finally, greater recognition needs to be given to the idea that a bridge might have a low level of national significance but be extremely rare and important locally.
HistoricBridges.org includes bridges that range from the rarest bridges on the planet to bridges that are just barely old enough to be considered to have any historic value. Most bridges on the website fall in between these two extremes, however.
HistoricBridges.org has developed its own rating system, the Historic Significance Rating (HSR) for bridges featured on the website. The purpose of the system is twofold. It is intended to help website visitors to sort out which bridges are extremely rare, and which are just a window into a period of history. It also to present a possible model for a future government historic assessment method that might deal with historic bridges and how to fund preservation projects more effectively. Please note that the findings of this system may not agree with what government systems currently assess a particular bridge. This is precisely the point of the new system, which is designed to be more flexible. It is based on the idea that a bridge is not simply "historic" or "non-historic" but rather can have a range of "historic value" as a historic bridge. The greater the value, the more important the bridge. The system includes an assessment of bridge significance on the local and national level. The scale is from 0-10. A bridge with a rating is zero is solely reserved for bridges that are not historic at all, such as a modern bridge. All other numbers refer to bridges that have at least some historic value, even if not much. Any bridge with a rating higher than zero in either category should be considered for preservation before being demolished. However, granted, a higher priority for preservation should be given to bridges with a higher rating. The HSR is designed to show that some bridges are more important than others, while also showing that bridges that are not as rare still indeed have historic value, and should still be considered for preservation. It also presents the case that bridges that are only locally significant are just as worthy for preservation as those that are nationally significant.
Lastly, the HSR rating system is a way for HistoricBridges.org to allow visitors to quickly learn which bridges are really special and which bridges are mostly just included as "representative examples of a common type" such as state standard concrete t-beams or steel stringer bridges, which often have HSR ratings below 5 to help keep them from competing with the spotlight that more significant bridges such as riveted metal truss bridges are worthy of.
