When is it time to pave a gravel road?
Paving a gravel road sure isn't cheap, and maintaining one also eats into your budget, especially as traffic increases. A new report—Economics of Upgrading an Aggregate Road—helps county and township governments make policy decisions for maintaining and upgrading gravel roads. Published by the Minnesota Department of Transportation (Mn/DOT) and the Minnesota Local Road Research Board (LRRB), the report will also help the public understand why counties and townships choose their policies and use certain construction and maintenance techniques.
The report, written by primary researchers Charles T. Jahren of Iowa State University and Greg Johnson of Mn/DOT, presents a method for comparing the cost of maintaining a gravel road with the cost of upgrading and maintaining a hot-mix asphalt, or HMA, road.
In their work, Jahren and Johnson analyzed an example scenario that included costs of a five-year regraveling cycle with yearly grading, plus typical costs for HMA surfacing, yearly HMA maintenance, and a seven-year seal-coat cycle. The economic evaluation used the present-worth method over a 30-year analysis period.
Cumulative cost trends
Using annual Mn/DOT State Aid reports, cost estimates, and interviews, Jahren and Johnson determined a typical county spending history for low-volume roads. They found that while bituminous roads have high initial costs, gravel roads cost more for ongoing, routine annual maintenance in later years. The graph of cumulative maintenance costs from one county (Figure 1) verifies that annual maintenance costs per mile for a gravel road increase with traffic volume. Each county can plot similar graphs based on its own data.
The researchers then identified the relationship between traffic level and maintenance cost. Though there were little data for either low-traffic paved roads or high-traffic gravel roads, the cost analysis showed an upward trend for gravel roads with increasing average annual daily traffic (AADT). In the AADT category with 150 to 199 vehicles per day—with nearly half paved roads, roughly the mid-point—bituminous and gravel maintenance costs were similar.
Recommendations: gravel to hot mix
Based on their findings, the researchers recommend that counties and townships give serious consideration to upgrading roads with traffic volume of 200 vehicles per day. They also believe it is reasonable to plan for the upgrade once traffic reaches 100 vehicles per day.
In some cases, an upgrade might be justified by maintenance savings alone, they say, such as when upgrading to a lightly surfaced road (seal coat). Lightly surfaced roads require a smaller investment than an HMA surface.
Other factors that cannot easily be assigned monetary value may also make it wise to pave a gravel road. Benefits include reducing dust, providing a smoother and safer surface, improving vehicle and driver efficiency, redistributing traffic, and potentially increasing the tax base. Some benefits directly affect county budgets while others have an indirect effect.
On the flip side, however, they note that after an aggregate road is paved, maintenance activities shift to those required for maintaining a higher level of service. Increased brush and weed control, traffic services, signage, pavement marking, snow and ice removal, and traffic control devices are typically needed for a heightened level of service. Anecdotal evidence suggests that snow and ice removal costs for paved roads are much higher than estimated.
Given these many factors, the report advises that local officials consider developing their own cost estimates for gravel road maintenance operations and checking them against their historical data. (In the report, estimates of primary gravel maintenance, grading, and resurfacing over a five-year maintenance cycle averaged $4,160 per year, an amount greater than the county historical cost results.) In cases where officials are confident of their cost calculations, they may use the estimate in place of the historical costs. Jurisdictions lacking sufficient historical data may estimate future maintenance costs, then compare them to historical data provided in the report. Any analysis must be modified, they added, to reflect individual maintenance or construction costs as well as project timing.
Download Report 2005-09: Economics of Upgrading an Aggregate Road (497 KB PDF)
Reprinted and revised from an LRRB research one-pager.
Related reports
- Report P2002-01: Best Practices for Maintaining and Upgrading Aggregate Roads in Australia and New Zealand (876 KB PDF)
- Transportation Research Record 1819, LVR8-1059: Minnesota's Experience With Thin Bituminous Treatments for Low-Volume Roads (145 KB PDF)