With today’s fiscally constrained environment, there are several cost-effective techniques that state and local authorities can use to reduce congestion. Last month I wrote here how synchronized traffic lights are a cost-effective way to reduce congestion on arterials. This month I want to detail how shoulders are a cost-effective way to reduce congestion on interstates or other expressways. States have an opportunity to convert shoulders to travel lanes during rush hour or twenty-four hours a day. Due to the high cost of acquiring land, converting shoulders to travel lanes is often a better choice than traditional expansions of urban and suburban expressways. These conversions are cost-effective and can be very effective in reducing congestion.
There are two types of shoulder conversion and three different uses. Most states convert shoulders to travel lanes during peak-periods. On Virginia’s I-66 shoulders are used inbound to D.C. from 6:15 a.m. to 11:00 a.m. and outbound from D.C. from 2 p.m. to 8 p.m. Some states have converted shoulders into travel lanes 24 hours a day by restriping the highway and narrowing general travel lanes from 12 feet to 11 feet. Los Angeles and San Francisco Expressways are good examples. Many states have 12 to 14 foot wide lanes that are unnecessary for today’s vehicles.
There are three shoulder conversion types. First, the extra lane is often used as a High Occupancy Vehicle (HOV) or High Occupancy Toll (HOT) lane where none existed before. One important goal of transportation planning is to offer cost-effective transportation options. HOV or HOT lanes allow vehicles with a minimum number of people or vehicles that choose to pay a small fee to use the lane. Buses and Vanpools can also use the lane for free.
Second, the extra lane is occasionally used as a busway. Busways are lanes that can only be used by buses. Typically HOV or HOT lanes are better options than busways because they handle more vehicles. However, in cities with existing HOV/HOT facilities, and/or a very high number of buses, and/or where a HOT/HOV lane is not appropriate, busways can be good fits. Busways also significantly improve transit service.
The third option is to convert the shoulder into a general-purpose lane. This is best in markets with existing HOT/HOV lanes or in cities with insufficient travel for a HOT/HOV lane. These general-purpose lanes can significantly reduce congestion and increase travel speeds.
What is the difference in costs between converting shoulders and building new lanes? Costs to retrofit existing highways to use the shoulder range from $250,000 to a million dollars per mile. The cost to build a new lane is $4,000,000 per mile or more in urban areas. Retrofitting existing highways is between one twelfth and one quarter the cost of building a new lane. An entire network of new lanes could be converted for the cost of adding one lane on one road.
Why are there not more shoulder conversions? Many states are wary of converting shoulders for safety reasons. However, several studies have shown no increase in crashes in any type of shoulder usage. A Federal Highway Administration study by The Transportation Research Board found no increases in crashes. Accidents on one part of the highway increased while accidents on the other part of the road decreased. This was due to decreased congestion on the highway. The increase in accidents was likely not caused by the additional lanes. The National Cooperative Research Project 369 found no significant increases overall in conversions in California, Virginia, and Washington. An ITE survey found accidents declined in a Virginia conversion project. That same study did show a 10% increase in accidents on one section of I-5 in California. Since this is the only statistically significant increase, it may be an outlier. This California section is a very short conversion with multiple weaving points. Highway sections with similar characteristics may not be good candidates for conversions.
Several states including California, Texas, Virginia and Washington now use shoulders as travel lanes at least part-time. Several other states are currently studying the idea.
States that have not converted shoulders should examine whether converting shoulders is appropriate. Every interstate highway in the country has a shoulder. While not every section of interstate or expressway is appropriate for conversion because of location or safety issues, converting a small fraction could improve mobility in a big way.