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Acceleration and Deceleration Rates used in Roadway Design Criteria


The values currently used for design criteria may be outdated. As a result road design may not be as efficient as they could be and may be more expensive to construct. In the current design criteria, a deceleration rate value of 11.2 ft/s2 is used. This value may need to be updated and increased due to the current vehicle fleet’s capabilities and changes in road wearing surfaces. Consideration of the appropriate deceleration and acceleration rates for driver comfort should also be considered with the vehicle fleet’s capabilities. Vehicles are now able to stop more quickly, because more have anti-lock brakes, better tires, and better overall technology. A 95th percentile value of 2.5 seconds for perception/reaction time (PRT) is also commonly used. In combination with the 11.2ft/s2 deceleration rate, this may be overly conservative. Similarly, vehicles are also able to accelerate more quickly due to engine improvements and other vehicle improvements. Changes in acceleration and deceleration rates will have a great impact on the design criteria used for designing roads. Locations most likely to be affected include interchanges and intersections. For example, entrance ramps could have a shorter required acceleration length and exit ramps could have a shorter required deceleration length. This would not only lead to a decrease in construction cost, but may also decrease construction time, since less would need to be built. Also, if the deceleration rate is increased, the braking distance and, as a result, the stopping sight distance would decrease. The profession should also consider if the object height should be modified from 2 feet and if 3.5 feet is still an accurate height for the driver’s eye. Our research will look at these values used in the design criteria and bring them up to date, in order to ensure we are constructing roads as efficiently as possible. Findings that recommend revisions to the acceleration and/or deceleration rates would then be considered for adoption in the AASHTO Green Book and then subsequently used in design by roadway engineers.


This research will determine appropriate acceleration and deceleration rates to be used in design. PRT, eye height and object heights will be evaluated and appropriate design values determined. The results are expected to be incorporated into the AASHTO Green Book. The values included in the Green Book should be based on current vehicle fleet operating capabilities. The values in the current edition of the Green Book are based on a prior vehicle fleet.


This research is important, because as our vehicular technology advances, so must our design criteria. We should use design criteria that is applicable to our current vehicle fleet. Designing to a prior vehicle fleet (and driver) can cause over design (additional costs) or under design (potential safety impacts). The benefits of updating the design criteria could be substantial. We would like to have the results of this research before the next edition of the Green Book is published.

This research topic was identified as a high priority by the AASHTO Technical Committee on Geometric Design, the TRB Committee on Geometric Design, and the TRB Committee on Operational Effects of Geometrics at their combined meeting in June 2018 from among a broad set of problems considered.

Related Research:

· Bauer, Karin M., and Douglas W. Harwood. Safety Effects of Horizontal Curve and Grade Combinations on Rural Two-Lane Highways. United States Federal Highway Administration, 2014.

· Marvomatis, Stergios; Psarianos, Basil; Tsekos, Pavlos; Kleioutis, Giorgos; Katsanos, Evaggelos. “Investigation of Vehicle Motion on Sharp Horizontal Curves Combined with Steep Longitudinal Grades”. TRB Annual Meeting 2015. Paper 1578.

· Edara, Praveen, et al. Evaluation of J-Turn Intersection Design Performance in Missouri. Missouri Dept. of Transportation, 2013.

· Gayah, Vikash V., and Eric T. Donnell. Establishing Crash Modification Factors and Their Use. Mid-Atlantic Universities Transportation Center, 2014.

· Hunter, Michael, et al. Reevaluation of Ramp Design Speed Criteria. University of Texas at Austin. Center for Transportation Research, 1999.

· Mehar, Arpan; Chandra, Satish; and Velmurugan, Senathipathi. “Speed and Acceleration Characteristics of Different Types of Vehicles on Multi-Lane Highways” European Transport, Issue 55, ISSN 1825-3997. 2013.

· Maurya, Akhilesh; Bokare, Prashant, “Study of Deceleration Behaviour of Different Vehicle Types” International Journal for Traffic and Transport Engineering. UCD 629.4:531.767. 2012

· Wang, Jun & Dixon, Karen & Li, Hainan & Ogle, Jennifer. (2005). Normal Deceleration Behavior of Passenger Vehicles at Stop Sign-Controlled Intersections Evaluated with In-Vehicle Global Positioning System Data. Transportation Research Record. 1937. 120-127. 10.3141/1937-17.

· Long, Gary. “Acceleration Characteristics of Starting Vehicles” TRB 79th Annual Meeting Paper 00-0980. 2000

· Deligianni, S.P. ... et al, 2017. Analyzing and modelling drivers' deceleration behaviour from normal driving. Transportation Research Record, 2663, pp.134-141.

· Yang, Guangchuan; Wang, Zhongren; Xu, Hao; Tian, Zong. “Feasibility of Using a Constant Acceleration Rate for Freeway Entrance Ramp Acceleration Lane Length Design” Journal of Transportation Engineering, Part A: Systems Volume: 144 Issue Number: 3 Publisher: American Society of Civil Engineers ISSN: 2473-2907

Based on NCHRP 400, “Determination of Stopping Sight Distances”:

· Most modern countries use a perception reaction time of 2 seconds instead of 2.5 seconds. This is closer to the 85th percentile of drivers than the 95th percentile used in the AASHTO policies.

· The deceleration rate of 11.2 ft/s2 is very conservative, and does not represent an emergency stop.

· The object height of 2 ft is very conservative given the current fleet of vehicles with high mounted center taillights and the driver’s inability to recognize the need to stop for a 2’ high object at great distances.

· The 85th percentile driver’s eye height is higher as crossover, trucks, and SUVs represent a larger segment of the vehicle fleet and safety standards have resulted in much taller vehicles.


Some of the tasks that could be completed in this project include:

· Completion of a comprehensive literature review. This review should include research on vehicle fleet composition, performance of the vehicle, advanced technologies and their presence in the vehicle, percentage of vehicles with the advanced technologies (i.e., anti-lock brakes and other performance/safety innovations related to braking and accelerating), tire / pavement friction based on current tires in production and typical pavement surface parameters.

· Evaluate the current research and identify which components need additional research. This could include:

o Tire performance / friction factors.

o Vehicle fleet.

§ Eye height and Object height (tail light height).

§ Which vehicles to use in design (is it still heavy vehicles/trucks and everything else)?

§ Acceleration and deceleration capabilities.

o Vehicle operator.

§ Acceptable acceleration rate, deceleration rate, and PRT.

§ Should the design parameters be modified if we have a high presence of “older” drivers? What would quantify “high presence”?

§ What if the vehicle does not have a human operator? Should this be considered and will alternative design parameters result from this (autonomous vehicles with reduced perception reaction times).

§ Should different acceleration/deceleration rates be used in different road locations (e.g., interchange ramps, intersections, and other locations)?

· Perform the research based on needs identified above and make recommendations.

· Propose new text based on the results of the research project for the next edition of the AASHTO Green Book.


All engineers and agencies involved in the design of roads will benefit from this research.

Sponsoring Committee:AKD10, Performance Effects on Geometric Design
Research Period:12 - 24 months
Research Priority:High
RNS Developer:Richard D. Wilder, David McDonald
Source Info:This problem statement was developed in connection with the June 2018 mid-year joint meeting of the AASHTO Technical Committee on Geometric Design, the TRB Committee on Geometric Design (AFB10), and the TRB Committee on Operational Effects of Geometrics (AHB65).
Date Posted:09/21/2018
Date Modified:12/31/2018
Index Terms:Geometric design, Design standards, Acceleration (Mechanics), Deceleration,
Cosponsoring Committees: 
Operations and Traffic Management

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