Why Do Some US Roads Have Two Different Speed Limit Signs?
The impact of state-controlled speed limits on traffic safety and policy complexities.
1National Maximum Speed Law
In 1974, President Nixon signed the Emergency Highway Energy Conservation Act, setting a national speed limit of 55 mph (90 km/h) in response to the OPEC oil embargo. A side effect was the drop in traffic fatalities from 4.28 per million miles in 1972 to 2.73 by 1983. The limit was raised to 65 mph in 1987 but repealed in 1995, returning authority to states.
2State Control Over Speed Limits
Since 1995, states have set their own speed limits, leading to political and legal complexities. Some states implemented fractional limits, decimal-point signs, or dual limits on the same road. Uniform Speed Limits (USL) apply one limit to all vehicles, while Differential Speed Limits (DSL) set separate limits for passenger cars and trucks, creating unique enforcement challenges.
3How Differential Speed Limits Work
DSL imposes lower speed limits for trucks and trailers. As of 2023, only 8 states (CA, ID, MI, etc.) still use DSL. For example, a 70 mph road for cars might have a 55 mph limit for trucks. 1990s data showed similar crash rates under both DSL and USL systems, suggesting speed limits alone don't dictate driver behavior.
4Traffic Interaction Studies
A 1964 study of 10,000 drivers found vehicles moving 10-15 mph slower than traffic experienced more interactions. In 2005, University of Arkansas confirmed a 10 mph speed differential increased interactions by 227%. However, these studies didn't specifically examine large trucks targeted by DSL policies.
5UK's Opposite Approach
In 2015, the UK raised speed limits for trucks over 7,716 lbs to reduce risky overtaking maneuvers caused by slow-moving lorries. In the US, the Owner-Operator Independent Drivers Association advocates eliminating DSLs, arguing roads are safer when all vehicles travel at similar speeds.
6Interstate Inconsistencies
Speed policies vary wildly across US states: Texas has 85 mph highways while Hawaii caps at 60 mph. This inconsistency confuses interstate drivers. In DSL states, truckers frequently face speeding tickets. The lack of federal standards creates enforcement disparities and economic impacts on freight transport.
7Traffic Safety Data
IIHS data shows traffic deaths increased 15% in the 5 years after states regained control in 1995. However, this stemmed from multiple factors including increased vehicle miles and infrastructure gaps. The direct impact of DSLs remains debated, as crash causation involves numerous variables beyond speed limits.
8Future Policy Directions
The DOT recommends states consider traffic density, road design, and crash data when setting limits. With autonomous vehicles emerging, tech-based solutions like dynamic speed limits may gain traction. The DSL debate highlights the ongoing tension between safety and mobility in transportation policy.
Original article: SlashGear — ABD'de Neden Bazı Yollarda İki Farklı Hız Limiti Var?