Setting Credible Speed Limits

NEW HAMPSHIRE DOT

ISSUE STRATEGIES TAKEAWAYS

➤ Legacy speed limits

➤ Setting credible speed limits

➤ Supporting local agencies

➤ Coordination between DOT and

local agencies

➤ Engineering, enforcement, and

educational approach

➤ Team effort

➤ Alignment of speed limits with

roadway context

➤ Working together with enforcement

➤ Support for local agencies

Background

New Hampshire DOT (NHDOT) is working to promote 

“reasonable and safe” speed limits for conditions and in a 

number of instances found that raising the existing speed limit 

was the appropriate solution. 

Establishing and managing credible speed limits impacts 

safety. Community safety often suffers when we reduce speed 

limits in reaction to an event without considering the context 

and operations of the roadway. Educating the public one 

town or one highway segment at a time takes a lot of effort 

and it is not always received well. However, when explaining 

why a speed limit should fit the character of the roadway, 

the NHDOT has been pleasantly surprised by the number of 

people that seem to “get it.” (B. Lambert, interview with the 

author, December 30, 2019).

The Challenge

There is a general consensus that where traffic speeds are a 

concern, lowering the speed limit is the perceived solution even 

if it is understood that the requested speed limit value is well 

below what is considered “reasonable and safe” for conditions. 

Decades of questionable speed limit practice have resulted 

in a network of speed limits that are not always credible with 

respect to actual conditions thus contributing to a culture that 

treats the speed limit as a minimum, not a maximum value. 

This culture is often reinforced through the actions of the 

courts where law enforcement is rumored to be forced to 

include a tolerance of as much as 10-15 mph over the posted 

speed limit to secure speeding convictions. (B. Lambert, letter 

to DOS/Chiefs of Police, November 5, 2019).

NHDOT has worked with a number of communities to address 

locations where posted speed limits were out of character 

for the conditions thus creating both safety and enforcement 

concerns. Table 1 shows the excessive speeds observed where 

posted speeds were ultimately increased.

Table 1. Spot speed study examples

Roadway

Speeds (mph)

Posted 85th Percentile Difference

US Route 3 at NH Route 28 40 58 18

Wakefield NH 153 35 49 14

Wakefield NH 153 35 46 11

Wakefield NH 153 35 47 12

Hancock, US Route 202 45 55 10

Hancock, US Route 202 40 56 16

Candia, NH Route 27 35 48 13

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Setting Credible Speed Limits

When NHDOT presents this information to communities, 

they also show how speed limits have a relationship with 

other traffic control devices. For example, required passing 

sight distance and curve warning signage are a function of 

the posted speed limit and mid-block crosswalks are not 

allowed where posted speed limits are greater than 40 mph 

(see Curve warning signage photo). 

NHDOT also emphasizes to communities that speed limits 

need to be credible for the character of the road, including 

horizontal and vertical geometry, lane widths, on-street parking, 

roadside amenities, adjacent land uses, and all roadway users.

A credible speed limit should result in voluntary compliance 

by the majority of drivers based on these criteria. They note 

that if there is a desire to have lower speeds, it can only be 

achieved if the character of the road is also changed.

Working with Communities

One thing that NHDOT always points out to communities is 

that by recommending an increase in the speed limit, they 

are not advocating that traffic speeds be increased beyond 

existing conditions, they are advocating that speed limits 

reflect the true nature of the segment so they are more likely 

to be respected as a reasonable and safe maximum.

NHDOT has reviewed—and where appropriate—increased 

speed limits within a number of communities based on traffic 

and engineering investigations. For example, within the 

communities of Wakefield and Candia, the posted speeds 

were changed from 35 to 45 mph.

Specific to Candia, records indicated that the 35 mph speed 

limit was established at approximately the same time as when 

traffic signals were installed (roughly 1967). Current practice 

would not consider the presence of a traffic signal as a reason 

for reducing the speed limit. The Bureau of Traffic conducted 

an engineering and traffic investigation with the MUTCD 

practice for determining credible speed limits being that they 

are set within 5 mph of the measured 85th percentile speed. 

NHDOT also used a web-based program produced by FHWA, 

USLIMITS2, to compare the results. This considered several 

other factors, including traffic volume, road character, and crash 

history to determine a recommended speed limit.

On review of the engineering and traffic investigation with 

enforcement, there was agreement that increasing the posted 

speed limit through the traffic signal controlled intersection 

to 45 mph would be more credible, and therefore more 

enforceable, than the current 35 mph speed limit. The new 

45 mph posted speed required that the nearby traffic signals 

adjust for “dilemma zone” protection.

In Wakefield, the local police chief struggled to enforce the 

35 mph speed limit. If motorists were stopped and ticketed at 

45-50 mph, they were essentially penalized for traveling at a

speed that was reasonable and safe for conditions; however,

if they were stopped and ticketed at 60-65 mph, a speed

that was considered marginally excessive, they were required

to appear in court and faced a much stiffer penalty due to

the fact that they were stopped at 25 mph over the posted

speed limit. The DOT worked cooperatively with local police

to complete an engineering and traffic investigation that

supported increasing the speed limit to 45 mph.

Prior to replacing the speed limit signs, the DOT evaluated 

the horizontal alignment signing and passing zones based 

on the proposed speed limit and determined that an 

additional 200 horizontal alignment signs were added over 

approximately 12 miles, arguably providing additional safety 

information for the majority of drivers1. 

Curve warning signage in Wakefield, NH. 

Image Source: Bill Lambert, NHDOT

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Setting Credible Speed Limits

Key Takeaways and Lessons Learned

• Legacy Speed Limits –There is a lot of attention given

to highway speed and related safety concerns. In some

cases, agencies are faced with legacy speed limits

which were lowered in reaction to a safety event without

consideration of motorists’ compliance, the roadway

setting, and multiple years of crash history.

• Culture – The collective challenge is to develop a

culture where the posted speed limit is recognized and

respected as the maximum safe and reasonable speed for

the subject highway segment, especially when there are

examples of locations where that is not the case.

• Engineering – The effort begins with engineering as speed

limits need to be credible for the character of the road,

including horizontal and vertical geometry, lane widths, onstreet parking, roadside amenities, and adjacent land uses.

Enforcement presents a different set of challenges. Speed

studies completed by the Department of Transportation

routinely demonstrate that the majority of motorists

operate at speeds in excess of the posted speed limit.

Education may be the broadest category and the least

defined as there are a number of areas where education

can be applied to address speed management.

• Team Effort – NHDOT recommends that effective speed

management be a joint effort between the State DOT,

Office of Highway Safety, Division of Motor Vehicles and

Public Safety, and the New Hampshire Chiefs of Police

with a focus on engineering, enforcement, and especially

education. There may be additional highway safety

advocates that could also be effective partners in this

effort, many of whom are identified as stakeholders in the

state Strategic Highway Safety Plan.

1 Lambert, William. Speed Limits and Setting State Speed Zones: Selectmen’s Meeting [Presentation]. Candia, NH, November 14, 2016.

Case Study 3: Noteworthy Speed Management Practices 3 FHWA-SA-20-079