Why are newly sealed roads still bumpy? Here’s an answer from the experts

Seal coating, shown here on Osos Valley Road, won’t get rid of bumps and other irregularities, but it is a cost-effective way to extend the life of a road until a complete repaving is done.
Seal coating, shown here on Osos Valley Road, won’t get rid of bumps and other irregularities, but it is a cost-effective way to extend the life of a road until a complete repaving is done.

In a recent letter to the editor, a resident of Los Osos expressed concerns regarding the quality of work on a recent roadway surfacing project on Los Osos Valley Road.

Other residents within the county are likely asking similar questions, such as, “What’s up with the County paving job? Why is the County spending money on paving, without fixing the underlying road imperfections?”

These are excellent questions.

There are approximately 60 miles of roads in the county receiving surface treatments this summer, including Los Osos Valley Road. These roads are being protected and improved by resurfacing with seal coat. The advantage of seal coats is that they are a cost-effective method of extending the life of good condition roads. The tradeoff for being cost-effective is that seal coating doesn’t completely smooth out existing surface imperfections.

Seal coats will not remove irregularities in the existing pavement surface.

A roadway’s pavement surface degrades over time from traffic use and exposure to the environment. Seal coats are uniquely designed to provide a protective, wear-resistant coating that prevents water from entering cracks, which is the leading cause of pavement failure. This type of surface treatment is meant to protect, preserve and prolong the life of the road. Only expensive grinding and applying a layer of new pavement will completely address existing bumps in the roadway surface.

Seal coating requires prep work. In the year prior to a scheduled seal coat, county crews prepare the roadway for treatment by performing crack sealing and pavement patching to ensure the integrity of the underlying roadway and maximize the life of the seal coat. Often those imperfections seen on the road after seal coating are the remnants of the prior years’ prep work. Crews are careful to minimize the amount of remnant bumps and ridges, but they cannot eliminate all of them.

Surface imperfections do not necessarily mean the underlying road is bad. Roadway pavement conditions are assessed and rated on a scale of 0 to 100; with 100 being a brand-new paved road.

On roads that are rated good — pavement condition above 60 — and do not have significant deficiencies, the main objective is to cost-effectively extend their life. Good roads with mild deterioration can be restored through surface treatments like seal coating. If left untreated, a good road surface can eventually turn bad. For unmaintained roads, pavement deterioration rates increase as time goes by. For roads that are rated poor — pavement condition under 60 — restoration options are limited to repaving.

The advantage to maintaining good roads through seal coating is this: It costs less to maintain them while in good condition rather than fix them when they’re in poor condition.

Repaving a poor road can cost around $300,000 per mile or more, while seal coating can be applied to a good road at one-tenth the cost — around $30,000 per mile. In addition to being less expensive than paving and also extending the life of the roadway, surface treatments reduce ongoing roadway maintenance costs in the long run.

What’s the county doing about poor condition roads?

There are nearly 1,100 miles of paved roads in the county system. Ending fiscal year 2013-14, there were 610 miles of good roads and 482 miles of poor roads. Five years later, there are 708 miles of good roads and 384 miles of poor roads. Thanks to significant investment from the county’s General Fund and the Board of Supervisors making roads a priority, the county has improved almost 100 miles of poor-condition roads by converting them to good condition roads, to be maintained as such in perpetuity.

There’s still a long way to go. The use of cost-effective methods such as surface treatment allows the county to stretch limited maintenance funds and balance extending the life of good roads while also improving poor roads.

Uncertainty in future transportation funding, including SB 1 (the state transportation tax), means costly but needed repaving projects may go unfunded, leaving only maintenance funds for surface treatments like seal coats and reversing the trend of improving poor roads.

For more information about the county’s Pavement Management Program or to see when your county road is scheduled for treatment, please visit

Joshua Roberts is transportation division manager for the county Department of Public Works.