The multi-year, possibly $100 million project to repave Newton roads is being driven by an objective measure of the condition of the roadways. It ought to be guided as well by an objective measure of how safe the roadway design is.

In software development, there’s a concept called “technical debt.” Technical debt represents the limitations that exist in the software because of design decisions you made out of ignorance or expediency, limitations that you’ll have to fix/eliminate at some point. You can apply the concept to our roadways, which have both condition debt — the limitations due to deferred maintenance — and design debt — the difference between the roads as they are and the way we wish they were, which will cost a bunch to rectify.

Anyone traveling on Newton roads knows that we have an enormous roadway condition debt. We haven’t been investing in maintenance, roads are in terrible condition, and the cost to rebuild is going to be dear. Thanks to very cool ScreetScan technology, we have an objective 0-100 score for the condition of every foot of the 350+ miles of road in Newton. 

We also know that maintaining roads above a score of about 80-85 is (relatively) cheap and that roads below the threshold have to be rebuilt, at a cost that increases markedly as the score goes down.

The city’s average rating? 62. Well below the maintenance threshold. It’s going to be wicked expensive to fix our streets but, if we follow a sound maintenance plan once they’re fixed, it should be reasonable to keep them in good condition. The difference between the healthy roads threshold and the actual rating can be thought of as our street condition debt.

We also know that we have a serious roadway design debt, we just don’t know it by that name. In neighborhood after neighborhood, residents complain of too high speeds. Cars and trucks driving too fast is not only unsafe, it discourages biking on and walking along and across our streets. Which drives people into their cars. Which, … you get the point.

It’s not an accident that cars drive too fast. The 20th Century, car-centric design of our streets promotes unnecessarily high vehicle speeds. When built, facilitating vehicle traffic and promoting flow was the main point. We know better now.

If only we could measure design debt as simply as we can, now, measure condition debt. 

Turns out, we can. Every stretch of road has a “design speed”: the speed at which the 85th percentile driver drives. The design speed reflects the comfort drivers have on the road, which is a function of the design elements. The wider and straighter the road (among other attributes), the more comfortable drivers are driving fast … the higher the design speed.

Every stretch of road can also be thought to have a “desire speed,” the speed that we as a community wish the 85th percentile driver drove. If we were designing a road from scratch, we could apply 21st Century learning and techniques and build a road that yields our desire speed. 

Of course, we’re not building our roads from scratch. The difference between the roads as they are and the way we want them is the design debt, which can be measure by the difference between design speed and desire speed. This isn’t an abstract notion. We can throw pneumatic tubes down, measure travel speeds, and determine the design speed. And, we can make political decision about how fast we want traffic to go on which roads. The difference between design sped and desire speed is an actual, calculable number. We could just as easily catalog the design debt for Newton roads as we have cataloged the condition debt.

The city has measured the condition debt of our roads and is clearly investing in paying it down. We need to measure and eliminate the design debt, too. Addressing the condition debt will make driving smoother. Addressing the design debt will improve safety and quality of life for everyone.